|The SRC X65 with its original box, now long gone.|
Wednesday, 20 August 2014
I've been using a 65ft inverted L with a 9:1 unun for some time. It was originally made by the Snowdonia Radio Company (SRC), but I have had to rebox it as the unun box cracked and filled with water.
With a new box and stainless steel bolts and fittings, the antenna works well from 20m – 10m, with coverage of 40m and 80m as well, although performance isn't fantastic on these lower bands.
The antenna is a vertical for the first 8m, supported on a fibreglass fishing pole. It then goes off at quite an acute angle down to the garden fence. The pole is held against a tree trunk with a couple of bungee cords and has a single earth stake. It is fed with RG213 coax with a 10-turn choke at the feedpoint.
What I did notice was that the antenna didn't work very well on medium wave, long wave or with LF aeronautical NDBs, so I thought about trying something.
The unun box now has a DPDT switch attached that switches out the unun and feeds the antenna directly. This has made a big difference to medium wave with signals romping in. The switch is waterproofed so all I have to do is go outside, flick it if I want to use the antenna on medium or long wave, and flick it back to engage the unun, which gives a better match on the upper HF amateur bands.
Without the unun I also get a better match on 80m, which is a bonus. Simon and SRC are no longer trading unfortunately, but you can build your own 9:1 unun quite easily.
While it is no Flag, Kaz or Wellbrook Loop, it does make a simple omnidirectional antenna for SWLs and hams.
|The circuit diagram - you can easily make this on |
a piece of PCB.
This has to be the ultimate in small antennas – one that will receive everything from VLF to HF (even VHF) in a package less than six inches long!
The antenna was conceived after Roelof, PA0RDT, had several attempts to make an active loop work in a city environment.
What he discovered was that the electric field from local noise sources was generally contained within his house. But, the magnetic field of noise sources was not, making weak signal reception at LF virtually impossible.
From that he decided that an antenna mounted outdoors that was receptive to the electric field rather than the magnetic field might be useful.
After extensive tests he said it became clear that at LF an active whip is effectively a “capacitance coupled to the electric field”.
Roelof added that in practice the “whip” can be tiny, such as a small piece of copper clad printed circuit board. Hence his Miniwhip was born. Roelof's practical design is only 100mm long and 40mm in diameter yet will receive from 10 kHz – 30 MHz.
It is an active antenna and that feeds the power to the antenna via the coax and a bias T circuit.
The antenna details and schematic are available on the internet for you to make one, but Roelof also makes them to order (you can email him at email@example.com). At the time of writing the cost was €48 including shipping within Europe.
The antenna and its amplifier circuit are built into a sealed grey plastic pipe leaving you only to connect a suitable length of coax via its BNC socket. You will need to provide a 9-15V supply, such as a small “wall wart” PSU. But it must be a “clean” supply as some of these can be electrically noisy.
Extensive tests with the antenna showed that it is very prone to receiving noise (which I had been warned about). In my shack it picked up all manner of interference, from switch mode power supplies to a low-energy light bulb. Even a TV on standby in a room 20ft away caused problems. To be fair, this was to be expected. Roelof says it is an excellent noise sniffer! He has used it extensively to investigate several local noise sources.
It is best mounted outdoors and as high as you can get it. In fact, I mounted mine on a telescopic fibreglass pole so that I could test it at different heights up to 8m.
|The tiny commercial version, direct from Roelof.|
Extensive use of ferrite chokes might also be useful. I found that my two PCs produced a lot of interference, which was picked up on my Perseus SDR receiver. Using my Icom 756 Pro 3 with the PCs switched off made a big different, especially on the LF/MF bands. Roelof says that grounding the shield of the coax before it enters the house or at the bottom of the mast is also important.
What happens is that local noise is received on the shield of the coax inside the house and travels to the antenna. By grounding the shield, the noise will "flow" to earth. This can make a considerable difference. He has also included a RF isolating transformer in the power bias T box. A jumper is used to select between isolated and connected grounds.
What also soon became apparent was that the higher I mounted it, the better the Miniwhip worked. In fact, at about 5m it almost matched the performance of my existing HF antennas, despite its tiny size.
Roelof says that moving the antenna from 1.2m to 4.8m could increase the received signal strength by up to 8dB on the lower bands and I would agree.
I tested it on everything from non-directional beacons (NDBs at LF), long wave, medium wave and all ham and broadcast frequencies up to 30MHz and it worked very well indeed.
Mounted on a fibreglass pole the antenna merrily received aircraft non-directional beacons down in the 300kHz range – signals that were considerably weaker on my main doublet antenna.
It was a similar story with medium wave stations, with the tiny antenna pulling in stations from all over Europe in August. My usual test is to see how well I can pick up BBC Radio Wales on 882kHz from Washford, Somerset, here in Norfolk. It did this with flying colours.
Moving up to Top Band (160m) and there wasn't a lot of activity, but I did pick up some DL and PA CW signals that were at least as loud as those received on my W5GI antenna over the roof.
It was similar performance on 80m (3.5MHz) where the little antenna worked reasonable well, but was down on a dedicated 80m antenna. Noise levels were a lot lower though. On 40m (7MHz) the antenna also picked up signals, but the signal levels were down quite significantly. Interestingly, in some instances, the overall signal to noise ratio was no different, so in terms of copying the signals there was little difference.
|My own version built (very) ugly style and now |
mounted in a piece of 40mm PVC pipe.
Roelof says that he has his mounted at 4m on a non-conductive mast to get clear of bushes in his garden, but he has also had excellent results in an open field at a height of only two metres.
The Miniwhip makes an excellent SWL antenna, as long as you spend some time calming all the various noise sources in your shack.
I even mounted it in the loft, where it worked, but again noise levels were higher.
In the interests of experimentation I also decided to build one myself from the plans I found on the internet.
The antenna is made on a piece of single-sided PCB, with the top half acting as the antenna and the lower half holding the circuit. I made my circuit board by carefully cutting away the copper with a Dremel-type tool. I then built the circuitry “ugly-style” (very ugly actually!) onto the board, using Superglue to mount some of the components securely with their wire leads being used for the interconnections. The end result was a little Heath Robinson, but did it work? The answer was a resounding yes.
As a test I also tried it on VHF and found it was a reasonably effective little antenna for airband and 2m signals.