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.
Principle of operation is that by varying the pitch of the windings it should be possible to obtain multiple resonances or improved efficiency on lower frequency bands of operation relative to unloaded wire.
It is easy to obtain two resonances by adding a more tightly wound helical section on top of a less tightly wound section.
It may be possible to obtain multiple resonances by creating ‘trapped helical’ either by interwinding capacitance or by adding capacitive loading or patches to closely wound section of helix, however the operating bandwidth is likely to be very narrow.
and the Chameleon range Chameleon V1
Both types of antenna use a similar style of construction, but would seem to operate in a subtly different manner.
First the DG7PE design
I have major doubts about the efficiency of this antenna, as the construction technique is likely to introduce large resistive losses in the windings, and dielectric loss in the GRP support pole.
The suggested use of a 9:1 Unun is also of concern, as I believe that this will also introduce national losses which will mask the true impedance presented by the antenna to the transceiver. I estimate that the total antenna gain is likely to be in the region of 10 to 16db relative to that of a ¼ wave vertical.
However it may be possible to apply variable pitch helical loading to a larger vertical antenna, especially when used in conjunction with an auto-atu. The choking effect of the wound sections could reduce the active length of the antenna at the higher frequencies, whilst improving the efficiency at lower frequencies.
I have now managed to borrow a DG7PE antenna (Click here to see a higher resolution version)
And here is the impedance plot when fed against 16 buried radials (No 9:1 Unun)
So rather than having multiple resonances as I first thought it actually has none - or at least only very small ones !
The special winding technique reminds me of a valve PA anode choke, which is designed not to have self-resonance on any band.
However it's still a mystery to me how a 9:1 Unun improves the match, other than by introducing additional losses into the antenna system.
The Chameleon antenna would seem to be constructed in a slightly different manner. As the Impedance plots produced by Array Solutions indicate a resonance at around 3.6MHz. However I'm still slightly worried about the very flat SWR curve and values of Rs, which would tend to indicate to me that there was a fair amount of resistive loss present in the system.
Carl from Chameleon antenna has commented as follows:-
Several hundreds of those antennas have been sold during the last few years around the world. They're used by the Military Auxiliary Radio System (MARS), Amateur Radio Disaster Services (ARES) and American Red Cross just to name few in the United States. They have also been used by the Haiti and New Zealand earthquakes rescue teams and countless ham operators around the world. The Chameleon Antenna are compromise antennas and they need to be used accordingly to the situations and needs.
On the following example they used the included 4 counterpoise wires supplied with the kit with the UNUN. The antenna was grounded on a grassy area. It is important to notice that the counterpoise wasn't hanging down but was kept straight with a nice tension provided by the tent stake. Trying to keep the counterpoise as high as possible from the ground will make a difference on the results. Here they used the tripod at about 40 inches off ground and the wires sloped down to the ground using the supplied stakes to anchor them.
The following plots were made with an AIM 4170C. First plot is from 3.5 to 15.5 MHz. Note VSWR in 75m band is very acceptable for most radios with auto tuner, VSWR is the red line, R is the orange line and X is the greenish line. Return loss is shown in blue.
Next sweep is from 14 to 30 Mhz VSWR is very low (red line) As you can tell this antenna system will work with just about any portable/mobile radio with a built in tuner.
I’ve now got a V1 and have played with it for a while.
The basic principle is an 8ft long wire radiating element, which is linearly loaded on the LF bands by means of a helical winding. This makes it ‘sort of’ resonant on 80m (which is subtly different from the DG7PE antenna). The linear loading coil is bunched into separate sections so that it doesn’t have self-resonances on any other bands. When used in conjunction with the supplied 9:1 Unun (Ruthroff type wound on type 2 Iron powder core). It has sufficiently moderate feed impedance, that it can be used on most of the amateur bands, in conjunction with a coax feed and 3:1 max type ATU (as built into in many current transceivers). Alternatively an LDG Z11Pro is suggested by Chameleon. When used with an ATU and ground mounted, performance is very similar to the Yaesu ATS 100 screwdriver antenna. On 3.6MHz and 7MHz the single band hamstick antennas are much more efficient.
Inside the Chameleon 9:1 Unun. Note that the secondary side of the transformer only has one connection to the centre pin of the socket (marked with a red band).
The antenna can be used in two configurations. As an elevated broadband antenna requiring no tuner. Or as an elevated, or vehicle mounted antenna requiring the use of a tuner.
In the broadband configuration. The antenna is mounted on top of an 8ft high tripod (not supplied). The antenna mounting bracket has a 9:1 Unun mounted below it and four sloping radial wires connected to the bracket. The radial wires are connected to ground at their ends by means of 6” metal tent pegs. Which also help to support the tripod and antenna. Each ground peg provides approximately 150 Ohms of ground resistance. The 9:1 Unun (Ruthroff Unun wound on type 2 Iron powder material) does not a have the ground side of the secondary connected to the plug. So the radial wires act as ‘passively coupled radiators’. The antenna is supplied with two clip on ferrite coke sleeves. Which are intended to be mounted on the coax just below the Unun. In order to reduce any common mode current on the outer of the coaxial cable. The two clip on ferrites provide about 50 Ohms of resistive choking impedance. However the antenna needs the outer of the coax to work as a counterpoise (as the counterpoise wires are not connected to the Unun). So the clip on ferrite chokes actually operate as a resistive load. Which help to mask any severe impedance excursions which may otherwise occur. The broadband match is therefore achieved my means of distributed resistive losses in the system.
I measured the radiated field strength from the antenna. By means of a test transmitter, calibrated power meter and remotely operated spectrum analyser and active antenna. Repeatability of measurements is within about +/-1dB.
When operated in the broad band configuration. The antenna efficiency is quite low. I compared it to a ground mounted Comet CHA-250 broadband antenna. Which about the same overall height as the V1 on a tripod mount. The performance of both antennas was about the same.
When the V1 is used as a ground or vehicle mounted antenna. Where it is fed against some sort of counterpoise or radial system. Using the supplied 9:1 Unun and a Z11 Pro auto atu at the base of the antenna. The performance on the HF bands on 14MHz through to 50 MHz is comparable to a similar sized Hamstick type antenna mounted in the same position... On 80m the performance is about 10dB worse than a Hamstick, and on 40m it is about 6dB worse than a Hamstick. I have repeated these measurements several times and consistently get the same results.
Interestingly Chameleon have just released the ‘Hybrid series’ which would appear to be an attractively packaged 9:1 Unun and radiating element. I would assume that this is of similar design to the 9:1 Unun used with the V1 and other antennas in the series as it would appear to be interchangeable with the earlier versions.
I have no affiliation with Chameleon, but I have previously discussed my findings with Carl, in order to ensure that my V1 was working as it should be. Their customer service is extremely good, and the products fill a void in the market place. Especially for folks who want an off the shelf compact solution to solve specific problems. There are more efficient and cost effective antennas out there. But I suspect that most folks, who use the Chameleon antenna, do not have many other alternatives, or anything else they can use as a reference.
So to summarise. On the HF bands it works about as well as a similar length of wire fed via a 9:1 Unun. On the LF bands it performs worse than a conventionally loaded antenna of similar size e.g. a Hamstick.
I’ve since tried copying the design using pound shop fishing poles and have got reasonably similar results. I've also bought a selection of second hand hamstick type antennas for a few pounds. Which I’m going to strip down and rewind to see if I can obtain better results.
More details to follow.
M. Ehrenfried – G8JNJ – 09/04/2012 – V1.5
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.
This information is now on the baluns page