An experimental omni-directional antenna for 70cm which has both horizontal and vertical gain. This is intended for use with our 70cm WEB SDR
Modelling suggested that a stretched 3 turn helix with a helix circumference of approx 1/2 wave length and an overall length of 1/2 wave at 70cm, and fed with a gamma match at the centre would offer reasonable gain, an omni-directional pattern and mixed polarisation. Adding more turns changed the propagation mode towards that of an axial mode helix. Less turns reduced the gain in both planes of polarisation, and caused an imbalance between the horizontal and vertical gains.
Next a plot showing the modelled gain in both planes, when mounted 10m above ground and fed with coax.
First attempt at a practical design - approx 300mm long 100mm between turns
Fixed the antenna to a short length of plastic tube in the workshop, and checked with a hand held fluorescent tube to see if the voltage distribution looked even between turns. Got the lamp to strike and then wound the power down to about 5 watts so that I could see the areas of the tube that remained lit up more clearly. Observed four distinctive bright spots of similar luminance (although it's not that clear in the photo) at each of the high voltage points.
Decided to check the difference in field strength between Horizontal and Vertical polarisation, by connecting the antenna to the VNA output and a small dipole to the VNA input. Held the dipole about 2 wavelengths away and rotated it between horizontal and vertical polarisation. Although this isn't the ideal environment for antenna measurements, the received signal levels on each polarisation are remarkably similar. Looks like about 8MHz bandwidth between 3dB points, not too bad for elements made from 2mm diameter wire. Larger diameter tubing would give a much wider operational bandwidth.
Final version built from 22mm plastic water pipe and 5mm copper car brake pipe.
Graph showing the measured input impedance and SWR.
So far so good. Everything seems to work as predicted, I just need a break in the weather so that I can try it outside.
I finally got some time to construct a 2m version of the Mixed Mode Helix. The intention is to try this out on our microwave WEB SDR.
The helix is about 1m long, 300mm in diameter with 300mm spacing between turns. The exact length of helix has to be trimmed for resonance (zero reactance), and the gamma match is then adjusted for lowest SWR.
I built this version using 5mm copper brake pipe and 22mm plastic water pipe with 90 degree bends ends and a Tee.
The pipe joints still need to be glued, and the whole assembly waterproofed and painted.
The antenna has a fairly high Q and moderately narrow bandwidth, so any flexing of the helix can change the impedance match quite dramatically.
The 5mm brake pipe I used is really a bit too soft for this size of helix, so I had to add an additional bracing section to keep it in shape.
The 5mm brake pipe I used is really a bit too soft for this size of helix, so I had to add an additional bracing section to keep it in shape.
Modelling produced some good plots, which seem to be representative of the measured performance.
When constructed as shown, the radiation pattern is predominantly Right Hand Circularly polarised at low angles of elevation.
Pattern is Omni-directional with very slight asymmetry due to the gamma match and interaction with the mast and feed coax.
To be continued
To be continued
I’ve recently bought quite a few RTL DVB-T RTL 2832U / Rafael Micro R820T dongles to use for various purposes.
Whilst experimenting with these devices, I’ve noticed that one or two have much higher levels of internally generated unwanted signals and broadband noise than the others.
Prodding around with a spectrum analyser I’ve found that there are three main sources of internally generated noise.
Plus received noise on the connecting cables which can be from any of the above, or external sources.
Looking first at the three internally generated noises.
The 28MHz oscillator is a fairly integral part of the design, so there’s not a lot that can be done to reduce these signals. Soldering the oval shaped metal can of the crystal to the PCB ground plane can reduce the level of some harmonics slightly, but I found that any improvement was hardly noticeable.
Wideband noise with spurs from the USB data lines and the on board 3.3v to 1.2v DC-DC convertor could be dramatically reduced by adding some metal screening around the RF input stage. I used some very thin brass sheet that I obtained from a model shop for this purpose, but some metal from a tin can would work just as well. I found that this was easier to implement if I removed the IR receiver, blue LED and associated surface mount resistors from the top side of the board. Adding a small patch of metal across the underside of the RF connector pins on the reverse side of the PCB also made a big difference. Note that placing the whole dongle inside a tin can didn't seem to help. The noise seems to be coupled directly between components on the topside of the PCB.
I decided to try fitting 0.1uF chip caps across all the main electrolytic capacitors and DC rails. None of these seemed to make much of a difference to the remaining spurious signals that could be observed with the antenna disconnected, but I thought it was worth adding them for the sake of completeness.
If these modifications are done carefully, the plastic case will snap back on without any problems.
All of these modifications made a significant reduction of unwanted signals. The next two photos show the ‘before’ and ‘after’ unwanted signal levels at around 480MHz. Note that these signal levels were with the dongle set for the maximum RF gain of 42dB. When used at a more reasonable gain setting of 30dB, all unwanted signals (with the exception of harmonics of the crystal oscillator) were usually at a level near or below the dongle noise floor.
Before the modifications
After the modifications
I also noticed that the 820 sticks have got a back to back SMD package strapped directly across the RF input. It may be worth checking to see if that has got popped by static if you have a broken one.
Once I had sorted out the internal noise sources I took a look at what external factors could be causing problems.
I noticed that when I connected the short RF fly-lead that was supplied with the dongle to the input, a lot of noises I’d just got rid of had returned. These were not detectable when the input RF cable was removed. The main problem was the poorly screened fly-lead.
Unfortunately the connector is a male MCX, which I didn't have in my junk box. So in the end I just ran a small length of thin PTFE 50 ohm coax with good screening from the rear of the connector to a BNC plug. A quick melt of the plastic case and the cable exits OK, and the case will fit back together again. Fortunately this solved the problem almost completely.
I decided to cut up one of the dongle fly-leads in order to see if I could figure out why it was so problematic. As soon as I stripped back the coax and plastic moldings it was obvious that there was hardly any copper in the non-overlapped screen, and a very poor termination at the TV connector end.
Fortunately it's easy to take apart the crimped connector at the MCX end and fit to some proper PTFE coax. So this is what I’ve now done rather than wire directly to the board.
In order to further improve the dongle screening and make them more robust. I made up this de-cast metal box for use on our uWave WEB SDR.
A photo of inside of box prior to fitting ferrite beads on the coax between the Dongle and a bulkhead RF connector., in order to help reduce the likelihood of RF current loops between coax screen and chassis. I had to characterise suitable ferrites for the best VHF / UHF choking impedance, as most clip-on types or similar don't do anything at these frequencies.
The USB screen needs a good low Z connection to the box metalwork and to the RF connector on the box. I found that if I didn't do this the screening was not as effective. Also any AC potential difference between the antenna coax and PC chassis resulted in current flow across the RTL PCB. This added multiple 50 / 100Hz noise sidebands to the LO.
The type of USB connector I used was a chrome plated A to B bulkhead version from LCOM ECF504-BAS
As part of this assessment I've also been testing to see if the limited dynamic range can provide adequate performance on the more crowded bands such as 7MHz. For these tests I have used a passive broadband vertical antenna in conjunction with an up-convertor and RTL820 dongle. I used SDR Sharp with various gain settings but no AGC selected. In order to find a worst case scenario with very high level signals I chose frequencies near the 9MHz AM broadcast band.
First 12dB gain - note the level of noise floor relative to the maximum signals - looks clean with about 60dB dynamic range
Notice the two diagonal traces from Ionospheric sounders gradually sweeping upwards in frequency.
21dB gain - note the rising level of noise floor relative to maximum signals - Start of severe intermodulation and reduction to 40dB dynamic range
34dB gain - note the very high level of noise floor in relation to maximum signals - very bad intermodulation products and only about 25dB dynamic range
Finally 44dB gain - note the levelling off of maximum signal levels and excessive intermodulation products
Here's another trace of the 7MHz amateur band with about 12dB of gain.
No problems this time with intermodulation products, as the maximum signal levels are a lot lower than those from the AM broadcast stations on 9MHz.
Just to prove that I wasn't cheating with the above screen grab, here's a wider view of the night-time spectrum, with high power broadcast stations either side of the 40m amateur band.
I also performed some two tone IMD measurements on the RTL DVB-T RTL 2832U / Rafael Micro R820T dongle.
The three screen grabs show in successive order.
1. 3rd order products with two carriers each at -3dB WRT 0dB reference line on SDR Sharp
2. As 1 but at 10dB higher level
2. As 1 but at 20dB higher level
I believe this demonstrates that if the SDR dongle gain is set appropriately, it’s perfectly possible to use them for reception of the HF bands with a simple block up-convertor.
Note that in all of the above cases I was feeding the block up-convertor directly from a large broadband antenna with no additional filtering. I have no doubt that the performance could be further improved by the addition of suitable band pass filters before the SDR dongle, if a greater dynamic range is desired on a particular band.
Here’s my suggestion of how to use multiple SDR dongles with a single HF up-convertor for a WEB SDR front end.
Here's a very quick to build transmit loop for 5MHz capable of handling up to 250w SSB.
It uses a 3m long length of LDF2-50 3/8" O.D. Semi rigid coax to form both the loop and tuning capacitor.
The LDF2-50 has a capacitance per unit length of 75pF/m). So the loop ended up a bit smaller than I'd hoped for. As I'd calculated the required tuning capacitance would be about 300pF for a 4m circumference loop. In the end it turned out to be something like 330pF with a 3.5m circumference.
By using something like LDF4-75. Which has a lower value of capacitance per unit length, or the larger diameter LDF4-50, LDF6-50 or LDF 7-50. It may be possible to further improve the efficiency and power handling.
The calculated usable bandwidth is around 3.5KHz. But I measured about 10KHz bandwidth between 2:1 SWR points.
With the loop just hanging on a upstairs door frame inside the house, and using 100w of SSB. I was able to work stations throughout the UK, and typically got 5&7 reports from stations about 200Km away. I'd estimate that the performance inside the shack was about 20dB down in comparision to my external 5MHz dipole at 15m AGL. I thought it would work a lot better if it was outside the house, as RF was definately being absorbed by the building structure and various items in the house (EMI).
The feed loop is made from a 1m length of coax inner.
Tuning is achieved by connecting the inner of one end of the loop to the outer of the other end. This uses the coax capacitance to bring the loop to resonance.
By sliding the inner of the coax over the outer of the other end it is possible to tune it over a reasonable segment of the band. You can lock-off the tuning by tightening up the hose clamp. Once you have got the correct tuning point. I'd suggest using two (or more) hose clamps in order to help further reduce the contact resistance at this connection point.
Once you have tuned the loop. You can optimise the SWR by either elongating or flattening the feed loop.
I also added a 1:1 current balun on to the feed coax in order to reduce common mode current on the feed line. This improves the loop balance, tuning stability and reduces noise pickup on receive.
When running 100w the loop current is about 40 Amps and voltage across the ends of the loop is about 3,700 Volts. It is noticeable that the first metre of the coax loop, at the opposite end to which the inner core is connected to the outer. Becomes noticeably warm after running 100W carrier for several minutes. This would seem to be where the majority of loss is occuring in the loop structure.
In order to ensure that the very high voltage that is present doesn't arc between conductors. You must strip back enough of the outer screen at each end of the coax,
This next photo shows what happened when I hadn't left a large enough gap, and used 250W of CW for a few seconds as a test.
I managed to get hold of some 1/2" diameter LDF4-50. This has the same capacitance per unit length as LDF2-50. But offers a higher breakdown voltage, larger surface area and much thicker inner conductor. I also found that it was much easier to work with, and held its shape better without requiring further mechanical support.
Modelling with EZNEC indicated that with the larger diameter outer screen of LDF4-50 efficiency should improve by 3 to 4dB in comparision to the LDF2-50 I used in the previous version.
Here's a photo taken whilst I was still trying to optimise the feed loop. The shape of the loop shown in the photo is a bit unusual, but it still provided a good match.
Here's a plot showing the SWR curve and useable bandwidth.
Here's a plot showing the calculated gain difference between my external dipole and the two loops.
When I mounted the LDF4-50 Loop in a clear spot outside the house. With the top of the loop at a height of about 3m. Performance was about 18dB worse than the external 5MHz dipole mounted at 15m above ground.
Further experiments at different positions confirmed a fairly consistent difference of 15dB. This was true even with the loop mounted in the middle of an upstairs room of the house. But sited well away from other conductive objects.
The EZNEC plot suggests that the gain difference should be around 18dB at an elevation angle of about 55 degrees (the likely angle of incidence over the measurement path). However I think I was able to obtain better results. Because I was able to orientate the loop for the highest signal level at the Hack Green Web SDR. Whereas the orientation of the reference dipole was fixed. Plus the dipole matching network could easily have introduced another 1 to 2dB off loss.
So these figures would seem to be about correct, and confirms that using 1/2" diameter LDF4-50 provides a noticeable improvement over the thinner 3/8" diameter LDF2-50 coax.
I was interested in finding out exactly where the losses were occurring. So I took a thermal image of the loop after running 200W carrier into it for about 5 Minutes.
There appear to be three hot spots.
Note that other areas of the loop do not seem to show significant signs of heating due to resistive losses. So I assume that most loss is due to the composition of the dielectric material.
Here is a circuit of simple HF Loop antenna suitable for QRP operation on all bands from 1.8MHz to 29MHz.
I originally built this for RX direction finding so that I could locate the source of some interference. I found that it also worked quite well on TX. Although the tuning is quite sharp on some bands.
The different frequency bands are selected by means of a four position rotary switch. This selects different values of fixed capacitors in various series / parallel combinations in order to achieve resonance on the required frequencies.
The capacitors are Silver Mica rated at 500v working, and the variable is a small air-spaced model. These provide a working upper TX power limit of around 15watts.
The capacitor that is formed from a short length of twisted PTFE insulated wire. Is used to adjust the highest frequency of operation on 28MHz. This also affects the frequency coverage around 21MHz when the variable capacitor is set to maximum capacity for the same switch position. Some twisting and untwisting of the wire may be needed to obtain the required frequency range. It is possible to use a switch with more positions and some additional capacitors, or use a variable capacitor with a higher value of maximum capacitance. In order to achieve a better overlap between each of the switched frequency bands. However the present arrangement works OK for me. The tuning is quite sharp and a large diameter insulated tuning knob. Helps to reduce the effect of hand capacitance whilst adjusting the variable capacitor.
The screened coupling loop is made from small diameter PTFE coax. The screen on the coax is disconnected at the mid point for a length of about 5mm. At the end of the coax feed loop. The inner and screen are connected together and soldered to the outer screen of the coax at the start point of the feed loop. You may need to experiment with the shape and position of the feed loop slightly. In order to obtain the best match over the entire operating frequency range. I had to fasten part of the feed loop so that it was in very close contact with the top of the main loop. So the feed loop eventually looked slightly triangular in shape. Once I'd determined the correct position and dimensions. I fastened the two loops together with heat shrink sleeving. The thicker sections on the loop are where I added some extra plastic tube in order to provide a mounting point for the support rods.
It may also be possible to build a portable version of this antenna. I'd suggest using satellite TV coax. Which is fairly stiff. To form a self supporting main loop. Fit 'F' connectors on each end of the coax and use mating sockets on the tuner box. The feed loop coax could be connected to a BNC plug and socket on the top of the tuner box. The loop can then be connected, or disconnected and folded up as required.
The control box was fastened to a short length of plastic conduit via a screwed adapter gland and fixing nut, to form a handle
Here is a picture showing the finished antenna.
I've just started evaluating various types and designs of Active Antennas for use on the Short Wave bands.
This is part of a project to measure the RF noise floor on various Amateur bands. Mostly on frequencies below 30MHz.
I have set-up an new page for this topic. Which can be found here
This is now on a separate page. Which can be found here
Here is a novelty way to use up old sets of Christmas lights - as a HF vertical antenna !
This animated sequence of photographs show the changes in current distribution along a 5m tall base fed vertical. Consisting of a string of Christmas lights with approx. 40 bulbs connected as four parallel strings of ten and fed with an auto tuner at the base.
The frequency in use increases gradually from 7MHz (most current at base) up to 50MHz (two current peaks with a null in the middle).
The lights are brighter where the antenna current is greater.
Note that the lamps have a non-linear response, as their resistance changes as they heat up. So the current distribution shown by the lights is not quite correct, but it does provide a good indication.
Flex radio 3000 – Very interesting radio. Hardware seems good; software is a bit flaky. You can download Power SDR and use it in demo mode if you want to take a look. http://www.flex-radio.com/
If I was buying again I’d go for the 1500 which is much better value in terms of bang for buck. Good points are - the panadptor tuning, second receiver with binaural audio (great for split working in DX pileups), brick wall DSP filters, the AGC system, the TX audio processing - all of which are excellent. The bad points the software bugs, the need for specific firewire interface boards (1500 is USB), the inconsistent layout and setup menus (right click on some items and you get to setup parameters directly, others items you have to navigate to via tabs), the internal ATU which is slow and has limited matching range, the poor noise DSP reduction and the missed opportunities to incorporate simple enhancements to standard features which would make best use of the PC based GUI format. Very much a work in progress and not a stable radio I would use in a contest. Flex radio support is very good, but you will almost certainly need it on more than on occasion.
The hassle associated with implementing a UK bandplan (especially if you have a 5MHz NOV) is a real problem.
The hassle associated with implementing a UK bandplan (especially if you have a 5MHz NOV) is a real problem.Flex released Power SDR V2.4.4 which had fixed a lot of issues. But it is still orientated towards the
Flex have now released Power SDR V2.5.3 which has added more features, and fixed some of the older ones. I was hoping that they would have resolved the issue of different Panadaptor scales being required for each band (due to the noise floor and signal strengths varying as you move from LF to HF bands). But for some strange reason they have only implemented band specific scaling on the waterfall display. Crazy !!!!
Hopefully Flex will eventually work their way through the backlog of feature requests logged on their website, but I’m not holding my breath.
Cross Country Wireless SDR-4 receiver
Used in conjunction with Simon Brown's excellent SDR-Radio http://www.sdr-radio.com/ and HDSDR http://www.hdsdr.de/. A few false starts due to a fault on the hardware. But it’s a very good value general coverage SDR radio. Performance above 2MHz is good but 160m tends to suffer from quite a lot of intermod interference from the medium wave broadcast band. Generally I’d say it’s a lot better performer than many HF RX’s I’ve played with before.
Funcube Dongle Pro - Just got my FCD up and running with SDRRADIO quite good fun tuning across various satellite bands around 1.4 to 1.6GHz with a modified active GPS antenna.
Here's some traces from a LEO sat with fast doppler shift. Ignore the central trace. It's the dotted slanted ones that are the satellite signals.
And another, this time showing several downlinks from a geostationary comms satellite. With a low bitrate data service inside the gray tuning bar.
More info on L-Band satellites can be found on the UHF satcom website http://www.uhf-satcom.com/lband/
When using a dual band 145/433MHz antenna on the roof. With no pre-amps or external filters (at the moment). Not much luck with the default gain settings, too much intermod from out of band signals. But after some experimentation I'm now getting good results on most VHF & UHF frequencies. With only very occasional bursts of interference.
LNA Gain +5dB
RF Filter 268MHz
Mixer Gain 12dB
Mixer Filter 1.9MHz
IF Gain 1 +6dB
IF RC Filter 1.0MHz
IF Gain 2 +3dB
IF Gain 3 +3dB
IF Gain 4 0dB
IF Filter 2.15MHz
IF Gain 5 +3dB
IF Gain 6 +3dB
LNA Enhance Off
Bias Current 11 V/U Band
IF Gain Mode Linearity
Sound card attenuation set to 30dB.
I played with various combinations of LNA, mixer and IF Gains, but these settings gave the best S/N ratios for UHF sat downlinks in the 250 and 437MHZ
bands. But I found that it was better to keep the mixer gain high at +12dB and the LNA gain low at +5dB (+10dB at the most).
I suspect that the LNA is not quite as low noise as it should be. So adding additional LNA gain doesn't do much to improve the overall S/N. But it does
increase the risk of introducing more intermodulation products.
It would be interesting to perform some RX noise measurements with different gain settings.
My personal opinion at the moment is to go for an external low noise amplifier at the antenna. Followed by a bandpass filter. Feeding the FCB optimised for best strong signal handling rather than absolute sensitivity.
The downside of these settings is that the DC offset is rather high with respect to wanted signals. Making tuning at the centre of the screen unusable.
Using a RTL2832U + E4000 USB DVB TV Dongle as a V/UHF SDR
I found this set of notes on G4FWR, Richard's website.
Some other notes can be found here
I acquired a suitable dongle from China for about £5 GBP and used software downloaded from the web.
In my opinion this is currently the best start-up guide for Zadig & SDR Sharp.
Don't bother loading the supplied drivers and note the big red warning note about which version to use.
This works quite well if the gain is set to a reasonable value (+30dB) and the antenna is not too large.
The RTL 2832U + Rafael Micro R820T USB dongle performs the best in terms of S/N ratio and Strong signal handling. 12dB SINAD for NBFM is somewhere around -120dBm which is reasonably good.
You can buy one from https://www.cosycave.co.uk/product.php?id_product=287
It works quite well even on 10m (providing you put a 10dB attenuator between the antenna and dongle).
With a roof mounted dual band VHF/UHF antenna of moderate gain. I can hear most signals that I'd expect. Plus a few that I wouldn't !
Overall quite good for a £5 GBP RX and certainly better than some of the scanners I've owned.
The antenna wires can either be separate wires or ends of loops. As shown in this example.
Each antenna wire can be separately connected to one of four possible points. RF feed A, RF feed B, a common point C, or nothing at all.
In this example the RF is fed via the A (green led) & B (yellow led) ports. Which are connected to the North and West ends of two delta loops of wire.
The other ends of the delta loops are connected together via the common point C (red led) or disconnected from each other (no led).
Physically the relay box is mounted at the centre of the roof near the chimney. The basic switching concept works fine, but the relays I used don't really provide sufficient isolation between switched and un-switched wires.
Physically the relay box is mounted at the centre of the roof near the chimney.
The basic switching concept works fine, but the relays I used don't really provide sufficient isolation between switched and un-switched wires.
The transceiver and screened multi-core control cable are passed through separate choking baluns mounted near the relay box. This is to help reduce any common mode current which may exist on these cables. Which may contribute to pattern imbalance, noise on receive and RF in the shack on TX.
The coax is connected to the relay box via a good quality 4:1 Guanella Current balun. This is to help provide further isolation and present an easier match to the remote coax fed auto-atu.
However the overall gain is considerably less than I’d hoped for, and much lower than my all band 90ft doublet.
I think one of the problems is the lack of height at the feed point, and the close coupling into the structure of the house.
I've now temporarily tried spacing the wires away from the roof. By using some tip sections of fishing poles. Which I have attached to the guttering by means of very large crock clips.
This has improved things slightly. But the performance is still less than I'd originally modelled.
More work required :-(
Still playing with compact loops after building my own as shown here I obtained a second hand AMA3 by Käferlein http://www.ama-antennen.de/ (no longer made) covers 13.9 to 30MHz at the at FRARS rally.
After an embarrassing exchange with the manufacturer. I discovered that the previous owner had written the wrong model number on the handbook !
It’s actually an AMA-6 covering 6.9MHz to 25MHz.
It has a huge variable capacitor inside (not butterfly unfortunately) which is worth what I paid for the complete antenna and controller alone. Nice heavy duty 1&1/2”diameter tubing, bolted directly to the end of the variable cap.
Although even this heavy duty construction still introduces some losses at the connection points. As can be seen in this thermal image (ignore the top and bottom caps - heat escaping due to thin plastic caps and mounting bracket - due to sun).
Whilst I was fiddling with the capacitor to try and reduce the minimum value of capacitance it could achieve. I found that the rotor contact left rather a lot to be desired. This caused variations in the best match that could be achieved when pulsing the tuning control backwards and forwards. So I cut up the capacitor and rebuilt it. So that there are now two sliding friction joints. One either side of the insulating back panel where the capacitor attaches to the loop. I can now set the tension on the spring compression washers which form the sliding joint. This has significantly improved the value of series resistance measured with the VNA and provides a more consistent match when tuning.
Another problem was RF on the motor DC control cable. I noticed this when I was looking at the radiation pattern with a fluorescent light tube. The DC cable was radiating as much as the loop. So I built a Bias T as designed by GM3SEK and described on his website (also published in the April 2009 issue of Radcom) into the feed point connector box. Phil AD5X also has a similar design on his website. This has solved the control cable RF leakage problem, and I now only need the one coax cable to the loop. This is a much tidier solution. Although I have to use a floating supply, in order to permit the voltage to be reversed when driving the motor in the opposite direction.
The motor speed control is really poor, just a 1W wire wound variable resistor in series with the DC feed. So there is not really enough torque to get the motor started when you do a slow fine tune. I think I’ll replace this with a PWM circuit instead at some point
On average when operating on 7MHz, I'd say the loop mounted at 2m AGL is about 6 to 10db down on a ground mounted 1/4 wave vertical. But the RX noise level S/N ratio is about 10db better than the vertical.
Here's some of the stuff I'm currently messing about with - please drop me a line if you have any interest in particular subjects.
70MHz transverter – I got hold of a ready built and boxed 28 to 70MHz G3WPO transverter for £5.00 at FRARS. The TX side works and gives 0.5W out but the RX is a bit deaf. I've now got hold of a circuit diagram so I should be able to do some faultfinding. All I need to do then to get on 4m is to build a linear PA. Either a £30 brick or an ex PMR PA strip with some biasing should do the job.
Mil Radio – Went to the War & Peace show in Kent looking for radio kit. All far too expensive, but I still had a good time. Some of the re-enactment displays were astonishing in terms of attention to detail. The recent improvement in sunspots has allowed me to work DX including Israel and the US on 10m using just the PRC320 manpack and 2.5m Whip. I've now got hold of two more scrap PRC320's which I have got working. One needed a major rebuild as the wiring harness associated with the turret tuner had been chopped through, when it was robbed by a previous owner to fix another unit. Strangely enough this one works better than the other two, even though it looks a bit tatty.
Here I am, braving the elements to work stateside on 10m USB whilst getting soaked with sea spray.
Information on this project can now be found http://g8jnj.webs.com/hfvariablepitchhelical.htm
This information is now on the baluns page