My Experiments with Mag Loop Antenna

Magloop by VU2ASH

Since long time was searching for a small but moderately efficient Ham antenna for limited space application. Magloop or Magnetic loop antenna is very suitable for this purpose. Let me state some basic concepts, which I presented during LARC Hamfest on 21-22 Dec 2019 at Hyderabad.

MAGNETIC LOOP ANTENNA

  1. Radio frequency waves
    Generated due to oscillating electrical signal in an antenna in radio frequency
    range. Consists of electrical and magnetic components at right angles to each
    other, hence called electromagnetic wave.
    2.Regular antenna from ¼ to full wave length.
    Evolution from simple dipole to full wave loop
    These antenna work by receiving electrical field, small current is trancduced in
    antenna, Rx amplifies it and converts to audio/intelligence.
WHAT IS MAG LOOP ANT

3.Mag loop work differently, they tranceduce magnetic portion of the wave, like a
secondary coil in a transformer and are basically LC tuned circuit and are not
affected by electrical interference.
Can be practically easily home-brewed from low 3.5mHz to high 30 MHz.

  1. It is a compromise small antenna, performance is less then a full size wire
    antenna on low HF bands.
  2. The circumference of magnetic-loop antenna as recommended should be less then
    0.1 wave length, if more, it will become an electrical loop antenna.
    However ARRL suggests circumference to be 0.08 wave length.
  3. There are equations and tables available to design this antenna, but
    experimental results do not always agree.
  4. There are lot of controversies and opinions on exactly how these antenna work.
    This presentation is based on my experiments and opinion.
    Advantages of Magloop antenna
  5. Mag loops are quite – not affected by electrical interference.
    2.They are small for given wave length, generally 0.1 lambda or smaller
    circumference say 2m for 20meter band. Occupying less floor space.
  6. Can be mounted close to ground, typically 1 or 2 dia off the ground.
  7. It is a bidirectional beam antenna, having max radiation in plane of antenna.
    Disadvantages.
    1.It requires high voltage high current variable capacitor.
  8. Has very low radiation resistance so construction requires care to minimise loss
    resistance.
  9. MGL have small band width typically 1%.

To build one you requires

  1. High voltage plate type capcitor suitable for desired power output OR
    Vacuum capacitor
    Any other high voltage type like Trombone,sliding disc or coaxial cable.

How to tune

  1. It is a high Q LC circuit use antenna analyzer to get lowest SWR.
  2. Listen to peak volume in Rx.
  3. Large dia conductor will give larger band width.
  4. For dimensions and values Tables and calculators available on net.
    5 .Calculations above give theoretical values. Actual construction affects the
    antenna performance greatly. Do not hesitate to vary the parameters and see the
    results.
MY EXPERIMENTS
  1. Coaxial cable loop RG58 with broadcast variable capacitor and torroidal
    secondary pick up winding for rX only for 40-20meter band.
  2. 3 mm dia wire loop 1meter dia, variable capacitor and wire pick up coil for 20
    meters.
    Worked ,quite ,low swr, small band width,good for RX and Qrpp 2watt due to
    capacitor plate spacing less.
  3. Coaxial LMR 400 ,3.3 meter (10 feet) circumference loop, home brewed 2 section
    wide spaced gang capacitor driven by 1 RPM DC motor,6mm dia copper pipe pick up
    loop 1/5 of main loop circumference. Capacitor switching arrangement to work for
    7.00 to 21 MHz.
    Good SWR 1:1.1 to 1:1.6 for different bands, band width about 50 Kc. Loaded up to
    25 watts and used with Ubitx.
  4. ½ inch copper pipe loop in construction.

Tuning

  1. Manual tuning- required location of loop nearby.
  2. Remote tunning- Various methods possible
    A. Receiver Volume – SWR not 1:1 for TX purpose
    B. Field Strength – Continuous transmission required during tuning, may cause PA
    overload at mismatched state, requires low output 1-2watt capacity,
    modifications to Ubitx.
    c. Memory locations – costly develop sketch etc.
    D. AGC based tune – Pre tune to peak and fine tune with remote relay tunner with
    TX on for minimum SWR.
    Developed the above last system With Arduino based Slow Fast tuning With The help
    of our Star programmer Om Dhiru VU3CER,by sensing AGC voltage and final relay up
    down movement of DC motor manually if required. Tested during Field Day. Gave 1 s
    unit less reception and 57 reports with Ubitx 10 watt on 40 meters.

Simple VHF fox

Our Pune Hams and Amature Club  ‘PHARC’ was planning a VHF Fox hunt and was searching for a suitable Fox . This gave me an opportunity to design and home-brew one for the club.

The requirements I considered were :-

  1. About 50-100 mw output crystal controlled transmitter with narrow FM modulation adjustable, and to be powered by 2 nos 18650 batteries 7.5VDC for good life.
  2. Audio signal to be programmable , and suitable OFF and ON timings.
  3. Compact layout and small transmitting antenna.
  4. To work in 144-145 Mhz range.

For point no 2 , I decided to use a small arduino pro mini to generate the CW audio tone massage and a cyclic timer to set the ON and OFF timings. Final sketch is given below.

First prototype was built on perforated board and housed in Indigo Nut metal box as shown below.

Second prototype was built on the PCB given below and housed in 3D printed boxes with batteries for a neat and clean look.

The set up worked wonderfully in last 3 fox hunt we had in the club. the schematic and the layout details are as given below.





PCB copper side
Parts layout

Note:- My old PCB software uses other symbols for component values like nF for uF for capacitors etc. Similarly in PCB layout part nos have been changed to accommodate part size. As such go by component values !!

Part Details

Coil L1 – 30 Turns close wound 3 mm dia ,22 swg enameled wire.

L2 – 9 Turns space wound 8 mm dia ,12 mm long, 18 swg tin plated wire.Tapped at 2 turns from cold end for collector connection J5 of transistor T1.

L3 – 4 Turns space wound 8 mm dia 12 mm long, 18 swg tin plated wire.

L4 – 3 Turns close wound 6 mm dia ,22 swg enameled wire.

Shunt jumper between J2 and J3 to provide supply to transistor T2.

J1 is connected to BNC female connector center and PCB ground to external shell if you are using non-metallic box enclosure.

Use final transistor 2N2222 in metal cap NOT plastic.

Use about 19 inches of 18 swg tin plated wire for antenna mounted on BNC male connector.

Mount sockets for only those Arduino pro mini pins on PCB which are used and shown on right hand side of PCB. Check ‘ Raw and Ground pins are aligned to Pro mini. Some components are mounted on PCB below Pro mini

Tuning and set up

After assembling the circuit on PCB, Without mounting Pro mini board on PCB, connect 7.5 vdc to the board.Temporary connect Raw and Pd4 socket of Pro mini on PCB through a 1k resister which is base connection of transistor T3 through R10.

This will cause T3 and T4 BD140 transistor to conduct and provide supply to rest of the circuit and Led D2 should light up.

Keep trimmer capacitor C4 half meshed up and check if your 12 MHz crystal is oscillating by monitoring on a HF receiver. Adjust C4 where the oscillation just starts this will give you good FM audio deviation on 144 + MHz. Check the frequency on RX and note it down. Your final VHF frequency will be 12 times this.

Connect a 19 inch wire to BNC connector center as antenna. Now tune your Station Receiver to 6th harmonic of your xtal frequency i.e. 72MHz +_ you should hear a faint cw carrier. Now tune C6 33 pF trimmer for maximum signal on your Receiver.

Now tune C8, 12 pF trimmer for maximum signal on 12 x your xtal frequency as noted earlier in 2 meter band . It will be between !44 – 145 MHz depending on the setting of C4.

After setting is ok remove temporarily inserted 1K resister from pro mini raw and Pd4 sockets, and dis connect the DC supply.

Do not forget to provide a small heat sink to the 2N2222 transistor. you can use a 8mm pc of 6mm copper pipe just increase its bore to fit metal cap of 2N2222.

Programming Arduino Pro Mini

There are various models available of Arduino Pro Mini. The one I used is as below. The PCB is designed for this configuration. If you use any other please verify the pin connections or modify the PCB as required.

If you are buying a new Pro Mini ,please solder the mounting and programming male pins as shown in top pic and boot load it.

The sketch which is to be loaded on the pro mini is as follows

// Fox controller by vu2ash jan 07 2014
int corPin = 2; // Fox trigger input can be used as switch or remote control
// should be set high, switch ON when low
long idInterval = 22750; //delay between id 22 seconds,
int speakerPin = 5; //cw tone on pin
int idPitch = 500; // 700 hz tone can be chosen
int relayPin = 4; //ptt switch relay
int MorseLEDPin = 13; // morse led indicator
int charSpace = 600; //space in charectors 300 for fast cw
int wordSpace = 700; //space in words 350 for fast cw
void setup(){
pinMode(speakerPin,OUTPUT); // tone output
pinMode(corPin,INPUT); // trigger presence
pinMode(relayPin,OUTPUT); //for ptt on off
pinMode(MorseLEDPin,OUTPUT); // tx morse indicator
Serial.begin(9600); //monitor on
}
void dot(){ // dot routine
tone(speakerPin,idPitch,100); //play 500hz tone for was 100us—
digitalWrite(MorseLEDPin,HIGH);
delay(100); //delay same as tone play 100us
digitalWrite(MorseLEDPin,LOW);
delay(100);
}
void dash(){ //dash routine
tone(speakerPin,idPitch,400); // 200 for fast cw
digitalWrite(MorseLEDPin,HIGH);
delay(400); // 125 for fast cw
digitalWrite(MorseLEDPin,LOW);
delay(125);
}
// set charecters in morse as per your choice massage,here it is set for //’TTTTTT DE VU2PHA K’
void d(){ //send charecter d
dash();dot();dot(); //-.. morse
delay(charSpace); //put space between next charecter
}
void e(){
dot();
delay(wordSpace); //word space seven dots
}
void v(){
dot();dot();dot();dash();
delay(charSpace);
}
void u(){
dot();dot();dash();
delay(charSpace);
}
void two(){
dot();dot();dash();dash();dash();
delay(charSpace);
}
void a(){
dot();dash();
delay(wordSpace);
}
void s(){
dot();dot();dot();
delay(charSpace);
}
void h(){
dot();dot();dot();dot();
delay(charSpace);
}
void k(){
dash();dot();dash();
delay(wordSpace);
}
void t(){
dash();
delay(charSpace);
}
void p(){
dot();dash();dash();dot();
delay(wordSpace);
}

Broad Band Hex Beam

pic3

VU2ASH home brew Broad band ,4 band Hex Beam 5/12/09

pic2

Ashok VU2ASH shack.5th dec.2009

Broad Band Hex Beam

Since I shifted my working QTH to Ludhiana, Punjab, in 1996, I was looking for a suitable antenna for my hamming. A Multiband antenna with good Front to Back ratio, moderate gain, light weight , easy to construct and preferably full size elements to avoid critical traps and complicated matching systems,  in fact every hams dream !!

In Oct 2008 while searching on net I came across this multiband Hex Beam by Steve G3TXQ and instantly got hooked up. He has developed two designs Classic, which has a narrow bandwidth and Broad band version, which has almost flat SWR throughout the band.

The BB Hex beam basically consists of:-

  1. Two full size wire elements, a dipole radiator and a reflector for each band i.e., 14 -18-21-24 and 28 Mhz. All mounted horizontally and properly spaced to avoid any interaction, on a common insulated pipe or pole structure.
  2. A centralized common feed to all band elements directly from single 50 ohm co axial cable without any matching stub or system.
  3. Whole structure is like an inverted umbrella with minimum wind restricting area (less than 6 sq feet) perfectly balanced, stable and light weight, about 4-5 kgs depending on the pipes you use.
  4. Requires 11 feet rotating radius only, 22 feet of clear space! Can be easily mounted on low cost TV rotator.
  5. For the performance specifications the antenna has
  6. Front to Back ratio comparable to a 3 element yagi.
  7. Forward gain comparable to a 2 element full size yagi.
  8. SWR 1:1-1.3 flat on full bandwidth.
  9. Excellent reports on air.
  10. Low angle radiation, suitable for DX.
  11. And lastly, Affordable to our pockets, my total cost was antenna -4.5 K + Rotator -3.5 K.

 

The basic configuration of G3TXQ BB Hex Beam, the M shape is Dipole Radiator at the top and bottom C shape is the Reflector.

pic6

There are various construction articles written on this well established antenna. You can choose any form of construction suitable for your availability. Below are the links to these articles please go through them to understand construction details.

Have a look at these few…

http://www.leoshoemaker.com/hexbeambyk4kio/general.html

http://www.on7ru.net/hexbeam.htm

http://www.karinya.net/g3txq/hexbeam/

and

http://www.mw0jze.com

hex-beam@yahoogroups.com
You need to be wary of the wire-dimension data published on ON7RU’s web site. The dimensions apply to **bare copper** wire, even though he shows his beam being built with insulated wire. Unsurprisingly he then finds the tuning is low and has to make major changes to wire lengths.

If you are building with bare copper wire use the dimensions from G3TXQ web site or Leo’s – they are the same. If you are using insulated wire use the modified dimensions shown on Leo’s site.
You can also try building BB hex for 40 m, if space permits at your end, many did

It is not a big deal as the antenna is not much bigger than a 3 element 3band yagi.
Profen designs can be found here:
http://www.hexbeam.de/reflectedw_data.html

I have enclosed a photo of my constructed antenna. Following is the list of materials and comments

  1. The base plate was made from 4 mm thick Aluminum plate 14 by 14 inches. Try to use at least 4mm thick plate ,do not use thinner plate as it will get deformed .The plate can be procured from your local engineering metal shop .Mark the center and positions of all holes for the clamps after the six center lines of the spreader pipes (supporting poles )are marked. For clamping the spreaders I used 1 inch galvanized heavy duty conduit pipe c clamps available at any electrical shop. Use two clamps per spreader.
  2. For mounting top central feed 1 inch pipe and bottom 1 inch mast I used 1 inch threaded ms galvanized four hole flanges. These flanges are available at plumbers shop. Use at least 6 mm thick collar flange normally used for steam line or heavy fittings. Both are mounted at the center and on opposite side of the base plate.
  3. For spreader the best material is fiber glass pipes. Do not use PVC conduit or water pipes, they don’t last!! Each spreader is 140 inches long. You require SIX of them. The construction recommended by Leo is given below. HOWEVER you  may not get these sizes easily. As such I procured  the standard available sizes in India as
  4. Six pcs of 21 mm by 17mm fiber glass pipes 2 meter long each.
  5. Six pcs of 16 mm by 10 mm fiber glass pipe 2 meters long each. The 16 mm OD pipe is inserted in to 21 mm OD pipe and clamped to make total length of 140 inches for each spreader.
  6. The central feed support 25 mm OD by 19 mm fiber glass pipe 2 meter long one pc.
  7. This is the only costly material of the antenna. About 3 k. You can get these pipes specially ordered from engineering material supply store. They are being manufactured in India at several places.
  8. Another specific requirement is the cavler cord which is used for supporting the structure. This cord should be non stretchable under tension. You can get good quality 3/16 inch Dacron rope and pre stress it before using. Do not use cotton or pvc cord.
  9. The dimensions and other construction details are detailed in the links given above

 

  1. Tuning is straight forward If you have kept the spacing between elements and sizes correctly, you will have correct SWR.pic5
  2. It is recommended that this antenna performs best with the height. But a height of 40 to 45 feet above GROUND level gives quite good results.
  3. So friends try it and let me hear your loud signals.

 

73s  and all the best.

Ashok VU2ASH

ashok.ashokdjoshi@gmail.com

 

Some experiments in Direct Conversion Receivers.

I have been in search of simple but acceptable performance Ham band receivers for our new up-coming hams with limited resources.
Started with simple Regenerative receivers, but they are bit critical to adjust .
Next in line was DC receivers, and sure they gave a much better and reliable performance.
I am going to give some of the practical designs which I home-brewed, with as much details as possible.
!. A simple single band DCR.
In this design I have used ready available components and based on simple single transistor VFO, varactor diode tuned.
In coming RF signal is fed straight to a double balanced diode ring mixer and mixed with same frequency VFO. Mixer output is intelligence carrying modulated audio signal. It is detected, amplified and passed through an audio band pass filter/limiter. It is further amplified in a suitable audio amplifier IC to produce audio in a speaker.
It is important to note that maximum signal gain is achieved in the audio amplifier stages in a DC receiver, as such using a sturdy enclosure to mount the DCR pcb is must to avoid micro-phony effect at high volume levels.
For good frequency stability use of stable polyester capacitors in VFO circuits is recommended.
Since no RF selective stage is used at the input, for good selectivity a proper tuned Ariel should be used. Alternatively a suitable band pass filter at the input can also be used.

This design is more suitable for low HF bands, that is for 80 or 40 meters, as making a stable VFO at higher frequencies is bit difficult.
The circuit diagram and the PCB layout is given below.
NEW DCRx

NEW DCRx pcb

NEWDCRx pcb bare

Few proto-types  were made . they all worked nicely.
Amogh vu3des made one in a biscuits tin box, here is its link:-http://amoghdesai.com/technology/electronics/ashdcr-direct-conversion-receiver-by-vu2ash/

2. Two Band 40-20 meter and All band 40-20-15-10 meter DC Receiver with DDS vfo.

To get good stability on higher amateur HF bands and to keep simplicity of direct conversion receiver , the receiver circuit was redesigned and a AD 9850 DDS VFO with at mega 328 chip control having band switching output and 16×2 LCD frequency display was used.A five pole LC filter with 50 ohm output termination was used at the output of DDS VFO to get a clean output without any unwanted generated harmonics.

In coming RF signal is passed through diode switched band pass filter and fed to the double balanced mixer/detector IC 1186N pin 1. DDS VFO signal is fed to pin 8 of the IC. Detected audio signal from pin 6 is fed to a single BC547   amplifier predriver stage. Output through a 10K volume control is fed to audio amplifier IC LM386 driving a 8 ohm speaker. Adequate rf and audio shaping bypass capacitors used to limit unwanted high frequency audio noise normally found in DC receivers.Resultant audio is crisp and clean.

Complete DCR was assembled on two home-brewed single sided  printed circuit boards. One for DDS VFO and other for receiver. Both the PCBs are sized and assembled in a Indigo Nut Case box with all the controls and speaker inside!!    Only external connections are 8 volt DC in and BNC antenna in.

20-40 DCR with DDS VFO. Assembled on veraboard.

20-40 DCR with DDS VFO. Assembled on veraboard.

20-40 DCR, inside view,bottom box connections- audio out and DC in on RHS and antenna bnc on LHS

20-40 DCR, inside view,bottom box connections- audio out and DC in on RHS and antenna bnc on LHS

40-20-15-10m DCR

40-20-15-10m DCR

40-10m DCR inside view.DDS VFO ON TOP pcb and reciever and band pass filters on bottom pcb

40-10m DCR inside view.DDS VFO ON TOP pcb and reciever and band pass filters on bottom pcb

Details of the two above printed circuit boards are as follows.

A. Receiver board

Schematic:- DDS DCRx-4 band pcb layout,   DDS DCRx-4 band schematic.

Band pass filter details coils wound on 10×10 mm IFT core slug tuned.Primary wound on 3 pin side, link on 2 pin side. with 40swg enameled copper wire.

Band          Primary          Link        Parallel cap      coupling cap

7 Mhz        14 turns         2 turns     150 pf              5.6 pf

14 Mhz       8                  2             120 pf              3.3 pf

21 Mhz       5                  2             100 pf              3.3 pf

28 Mhz       4                  2             82 pf                 3.3 pf

High capacitance tuned circuits intentionally used to reduce out of band interference,builder may use his own choice.

B. DDS VFO board.

Schematic – DDS VFO With Band switch schematic , Pcb lay out – DDS VFO With Band switch pcb layout

Arduino sketch for programming atmega 328 p –

AD9850_LCD_ROTARY_IF__WITH_1_HZ_RES_ASH

Above sketch can be modified if you want to build only two band version of your choice.

Testing and tuning.

Assemble the DDS board first. Carefully recheck the pin connections of all ICs .Use IC socket for 328 p. Before inserting ICs , LCD display and AD9850 module power up the board and check +5 volt dc at 7805 output and on Vcc pins on ic base LCD and AD 9850 module. Insert all modules and power-up, you should see 7,000.000 MHz displayed on the LCD with 1000 Hz tuning step. Adjust the contrast preset near LCD mounting for proper display.Each push of encoder push button changes the tuning rate as 1Hz-10Hz-100Hz-1000Hz-10KHz-100KHz-1MHZ and back to 1Hz. A push on Band switching push button will change the band as- 7.000-14.000-21.000-28.000MHz and back to 7.000 Correspondingly you will get +5 volt DC on band out pins of 328p. This +5vdc is used on diode switch input pins on receiver board to select the required filter. Verify the correct output rf signal from the DDS VFO on a separate station receiver.

Now complete the receiver board .Connect +8 VDC, +5 VDC and earth on both boards properly.Connect VFO out to capacitor on pin 8 of IC 1186N .Connect volume control,speaker and antenna input and power up both the boards. You should hear audio sig in the speaker. Use a standard frequency generator and feed mid band rf signal at or near the antenna input and tune the filters for maximum output. That”s all. Connect your regular tuned antenna and start listening.. You will have to practice a little to tune proper ssb signal on a DCRx for LSB or USB reception.

A word of caution do not give more then +8 Volt DC at the input to 7805 regulator if you are not using a heat sink on 7805 other wise it will get very hot..Use a suitable series dropping resister if you wish to use higher voltage.

A 1:1  positive print of both the PCBs are given below, you can make your own PCBs by laser print heat transfer method.An excellent article for this is there on Amogh vu3des blog.

4 Band DCReciever board 110x52mm ,  DDS VFO With Band switch positive pcb .

It will not be out of place to say thanks to Amogh VU3DES for providing me details of DDS VFO, which was then modified by me to suit the DCR design, and off-course the energetic members of Pune Hams and Amateur Radio Society who pushed me to complete this project. The PHARS is also planning to make these PCBs available to interested hams.

Happy listening. 73s

Ashok vu2ash

.

No Frills Slim Jim for VHF

An Outdoor NO- FRILLS Slim Jim for VHF. To access Ludhiana Repeater from my qth 140 kms away, was looking for a suitable VHF antenna with following properties: 1 . An Omni-directional antenna to get Repeater as well as local hams.

  1. Better gain then other Omni antennae e.g., Ground plane, 5/8 GP, J pole.

3 . Easy to construct, reasonable life for out door use and economical as well. For all these requirements slim Jim fits nicely. It has 3 db gain then a GP, has lower angle of radiation for longer coverage, requires no ground plane radials etc and can be easily made from whatever conductive materials you have !! Basic configuration of Slim Jim is given below. The wave length in Meters for any frequency ‘F’ in Mhz can be calculated as Wave Length (meters) = 300/F (Mhz), considering Velocity Factor of 0.95 for Aluminium pipe, other lengths are Lambda/4 = 71.248/F in meters, Lambda/2= 142.496/F. Gap = 25 mm, S (spacing) =30mm. Feed point will be about 100 to 115 mm from bottom for 50 ohm coaxial cable, to be adjusted for proper SWR.   Building Details. For my build I used 6 mm dia TV antenna aluminium pipe. It is available at local hardware material shop in 10 -12 feet length ( 3.04- 3.7 meters approx), costing about 30 ohms per length then. You will require following materials. !. Aluminium pipe 6 mm dia, wall thickness about 0.3 -0.4 mm, about one length 10 feet (3.04 meter)to be cut in to three pieces as per details given and as per your chosen frequency.

  1. Aluminium flat strip of width 10/12 mm and 0.5 to 1 mm thick about 300 mm long for making shortening clamps OR a one foot (300 mm) long piece of the same above pipe to be cut, split and flattened to make a strip for making clamp.
  2. White PVC conduit pipe 1 inch dia (25 -30 mm) 1.5 meter long one pc for slim jim assembly and mounting. Please DO NOT USE GREY OR BLACK COLOUR PVC PIPE. It absorbs RF.

4.Co-axial cable RG58 50 ohms (not longer then 40 feet) For connecting antenna to your rig. You have various choice, a PL connector mounted at the antenna feed point, a small pig-tail of RG58 with PL connector at other end to connect longer RG213 cable ( if required). However it is preferable to have minimum numbers of connectors in the antenna feed lines to avoid connector losses at VHF or UHF.

  1. Cable ties white 100 mm long, M3 screws and nut with washer of SS, Brass or galvanized MS as last choice, M4 screw 50 mm long with nut and washers and one 3.5 mm X 12 mm self tapping screw.
  2. One 4 inch by 8 inches sun-mica board, used for house hold electrical switch board box top, cut and used in Slim Jim assy as insulated mounting!!

Assembly details are shown below for 145.00 Mhz. For other frequency you can calculate the lengths and use.

  1. Make the four clamps first. Two for top and bottom shortening of pipes and two for coaxial fixing on pipes as per the shape shown. Drill suitable holes for tightening screws. The clamps should slide over the element pipe when screw is loose and lock in place when it is tightened.
  2. Cut following sizes from the sun mica board- A. Two pieces of 35mmX 60mm for spacers. Mark the centre. Place the two pieces of 6 mm pipe equal distance from centre on 60 mm side. Drill two holes adjacent to each pipe of 3.5mm dia for cable tie locking. Drill one 3.5 mm hole at the centre. (see the picture)
  3. Cut one piece of 50mm X 100 mm from sun mica board and drill 8 holes as shown in picture for mounting the gap adjustment portion of the antenna.
  4. Cut 3 pieces from 6mm pipe A. Lambda/4 + 12mm(clamp width) –one pc. (0.491 met + 0.012 =0.503 met for 145.00 Mhz). B. Lambda/2 + 12mm – one pc.(0.995 met for 145.0Mhz).C. Lambda/4 +Lambda/2 + 2 X clamp width + Gap- one pc (0.491+0.9827+2X0.012 +0.025 =1.5227 met for 145.0 Mhz).

Cut one pc 40 mm long and split one side to make a tight sliding sleeve on pipe A for adjusting the gap for SWR and reactive impedance matching.

  1. Lay down on floor all the pipes as per picture and fix sun mica boards with cable ties and fix the both end clamps so that the internal lengths of pipe between clamps as per our basic calculated sizes.
  2. Fix the whole assembly on the PVC pipe by a 4mm screw bolted through the PVC pipe and by a 3.5 mm self tapping screw through the centre spacer, as shown in picture. DO NOT forget to insert coaxial cable joining clamps before fixing bottom shortening clamp.

TUNING.

  1. Mount the whole assembly in a clear spot on a terrace or open space so that no metallic objects are there for at least 3 meters around the antenna. You can temporarily mount the antenna by inserting PVC pipe in to sand or earth filled bucket.
  2. Connect the coaxial cable SHIELD to short element side clamp and CENTRE conductor to long element side cable clamp. Position the clamps about 110 mm from inside of bottom shortening clamp. The SWR adjusting sleeve in the gap should be in fully inserted condition on bottom element.
  3. Use a Antenna Analyser or your handy with a SWR meter in series and at low power level feed RF to the antenna. First adjust the coaxial feed clamps up or down for minimum SWR and then fine tune for a minimum by adjusting the gap by sliding the adjusting sleeve .
  4. Very accurate setting of SWR and minimum capacitive reactance setting (desired) can be done with Antenna analyser.
  5. For better performance of the Slim Jim , a common mode choke should be used just below the antenna feed. You can make one by close winding 9 turns of feed coaxial cable 100 mm below the bottom clamp on the PVC pipe mast itself (as shown in picture), or use  4 ferrite clip-on beads on cable.
  6. Now erect the antenna in suitable high spot by clamping the PVC pipe to a suitable metal mast. Making sure that the metal mast is at least ½ meter below the bottom of Slim Jim.
  7. It is observed that as you lift up any antenna above ground the tuning frequency tends to go up few Kcs, due to ground effect, as such you may have to re-tune and re-check the SWR when fully erected for optimum results.

Happy home brewing.

Basic configuration

Basic configuration

Feed connection and coaxial choke

Feed connection and coaxial choke

Slim Jim construction details

Slim Jim construction details

Hfi BITX building and corrections

Hi I have home-brewed the BITX 40 pcb given to all delegates by Farhan at HFI 2014 at Hyderabad. However I made it for 20 meters and is working quite good. Giving below some tips for your successful build :- 1  Decide the band you want to build. 20 or 40 m. 2. For 40 m use 4.000 to 4.915 Mhz xtals as supplied in your bought kit. For 20 m use you can use 10.000 Mhz or 11.000  Mhz xtals. 3 .Get 10 nos chosen xtals and choose 4 xtals within 100 Hzs for making xtal filter. If you are lucky you may find one with plus or minus 500 to 700 hz  of chosen frequency. Use Minus xtal for USB on 20 m and plus xtal for LSB on 40 m. Don’t worry, you can pull the frequency with the circuit given but it will be easier this way. 4  Assemble the xtal osc circuit on the pcb first, temporarily put a jumper to ground from L5 end of xtal and check each xtal ,mark them and note down its osc frequency with a frq counter or station rx with digital read out or evan use the rtl dongle and monitor/read the harmonic frequency and divide it by harmonic no to get the basic xtal frq.( a novel idea from swl Amogh). Now choose the xtals as above. 5 Corrections on PCB and schematic a- Ground the T1A  three coil join connection ( shown on the left hand side of T1a on schematic ) b- Omit R69 50R ; C32 0.1 c- Change  R29 to 10 R ; C73 to 0.001; R64 to 10 R if u want more audio; if you want total varactor tuned vfo without using 60 pf variable capacitor as shown in schematic then change  C59 and C60 to 0.001 poly, C56 to 0.001 poly; D14 to BB105 and use 5 K preset for R55 to set the tuning limit. Use small IFT can type slug tune shielded coil for L4 to improve stability. Change R45 to 100R; R41 to 22 R. 6. The VFO frq will have to be set as per your xtal used as for 40 m- xtal frq (lsb) + Vfo = 7.000 to 7.200 Mhz for 20 m- xtal frq (usb) + vfo = 14.000 to 14.350 Mhz. For 20 m that is 4 to 4.35 Mhz I used 70 turns of 36 gauge enamel wire on slug tuned ift type former overlap wound for half the length of former. For 40m 3Mhz vfo you may have to increase the turns. Use minimum value for C58 to get full range on varactor. 7. For input tuning of rx L1 to L3 , again suggest use slug tuned ift can type former. I used two pole filter for 20 m with 15 turns primary 36 swg and 3 turns 36 swg link. Both primary with 68 pf parallel capacitor and poles coupled with 10 pf. Similar set up can be used for 40 m scaling the values . 8. The design can be converted to two band 20/40m transceiver, working on it.

9. Will publish details of my assembly and interconnections in latter blogs.

10 th dec 2014.Mumbai

Adding a DDS or Si570 vfo

For making the Bitx in to a multiband rig, you require a stable scalable vfo. There are several plans available for making a DDS vfo with a PIC controller or Aruduino controll. I picked up one DDS at HFI with PIC . It has a built in IF shift of 10.000 Mhz for vfo frequency display on LCD for 3.5 to 28 Mhz ham bands. That’s why I chose 10.0 Mhz IF xtals.

The DDS output is connected to Q8 base via a 0.1 cap at R42, R43 and C43 junction, thru a suitable attenuator. While operating with DDS the internal vfo oscillater Q11 and buffer Q12 are switched off by removing +12v supply jumper.

Next thing will be to add one switchable two pole input filter for desired band, I chose 40m , at the input ,coils L1 to L3., and you have a two band version of Bitx !

Will give details when I go back to Pune. Some snaps of my assy during construction are given below.IMG-20141206-WA0025 IMG-20141206-WA0027

22 nd Jan 2015, Pune.

 Continuing on the project have done some modifications and added various boards in the cabinet, mainly :

– PA unit

It is my old tried out design, used in my home brewed ASH3 multiband ssb/cw Trx, 14 Mhz DCR DSB Trx, Softrock 6.3 multiband SDR Trx and lastly Softrock Ensemble 40/30/20 m RxTx, using two 2SC1307 transistors in push pull finals, 2SC1969 as driver and 2N2222 as pre driver. 2SC1307 has a Pd of 25 watts each at 50 Mhz, however here it is intentionally de-rated to produce 15-20 watt only in push-pull as a safe guard for possible mis-match !

The PCB is home brewed and contains 20/40 m relay switched Low pass filters and TX/RX antenna switch relay. Resting current on Tx is about 180 ma and 2.0-2.5 A on voice peak at 12.8 vdc.

PA pcb, LPF and antenna switch relay on the LHS

PA pcb, LPF and antenna switch relay on the LHS

Mounting of finals on Heat sink

Mounting of finals on Heat sink

– Rx input filters

Two band pass filters of two slug tuned coils are used for 20/40 m. Diode switching is used for the band selection. Filters are assembled on a veraboard and mounted on Bitx mother board.

A DPDT switch mounted on front panel used to switch DCV to PA LPF relay and rx diode switch.

Rx input filters 20/40

Rx input filters 20/40

– AGC and S meter.

As there is no gain control in Bitx rx, strong signals over load the audio amp and becomes unpleasing listening.

A simple but effective design was found on YC3LVX blogspot. It samples the audio from product detector to produce AGC and input for a 50 ua S meter.

Derived Agc is applied to the first IF transistor Q3 base . The R13 2k2 resister is disconnected from collector and agc voltage is fed instead through R13. YC3LVX recommends to remove the grounding 1 K resister, however I left it in place.The auto gain control is quite effective, but a little more agc delay is desirable. Will do some experimentation later. Agc varries at 5.0 vdc at no signal to 1.6 vdc on strong signal.

AGC veraboard attached on RHS to Bitx pcb. Extreem right middle is Bitx Rx/Tx  switch relay

AGC veraboard attached on RHS to Bitx pcb. Extreem right middle is Bitx Rx/Tx switch relay

Final assy

Final assy

Front view. Volume control and band selector switch on LHS, Two push buttons above DDS tune knob are Band select and vfoA/B select.

Front view. Volume control and band selector switch on LHS, Two push buttons above DDS tune knob are Band select and vfoA/B select.

– VFO/DDS control

I am using nicely made HF dual DDS vfo by vu2 ptr om Thyagu. It has a pre-programmed pic with display adjusted for 10 Mhz IF.For 7 and 14 Mhz it has following configuration:-

10 Mhz  IF (USB) + 4 Mhz DDS VFO  = 14 Mhz USB signal,

10 Mhz IF (LSB)  – 3 Mhz DDS VFO   =  7 Mhz LSB. signal. This required the carrier oscillator BFO to tune to both USB and LSB frequencies. This was done by replacing 22pf trimmer capacitor C45 ,by a BB105 varector and a 100pf series capacitor and retuning the L5 inductor. DC bias was given to the varector through 100 K resistor, so that with 0.0 Vdc bias BFO xtal tuned to LSB with residual capacitance and with 9.0 Vdc regulated bias pulled to USB frequency !

These bias again switched from same DPDT switch on the front panel. For some Xtals a switched 47 K preset may be required.

Some day I will request om Thyagu VU2PTR to re program the PIC to have high level mixing for 7 Mhz to avoid BFO switching.

– Cooling fan

Also added a temperature controlled  cooling  fan to the PA heat sink. However there is no space to mount the home brewed  control circuit inside the box !!

Now going ahead with the testing of the set up and will update the results soon.

Happy home brewing.

My first post in blogging world!

Hello everyone,

This is my first step towards blogging and amazing world of internet. Till now I’ve limited myself to mails and orkut. I’m venturing out inspired by my daughter-in-law’s food blog.

This will not be a food blog though. This will be more about TOOLS and HOME-BREWING!! I like playing with tools and assembling things to make something useful. I am an amateur radio operator. Since then my interest in tools has been on a high. So is my profession as well…I hope to meet new people with similar interests…

Let’s see what blogging has in store for me.

Best,

ADJ