HF Receiving Aerials and an Aerial Analyser
In the last of his 'mini-reviews', for now, Keith Rawlings is looking at a pair of fascinating HF receiving aerials
In the last of his 'mini-reviews', for now, Keith Rawlings is looking at a pair of fascinating HF receiving aerials, the Moonraker GPA-RX and X1-HF models.
Welcome once again to Aerials Now. When I found out that two of the items I would be receiving for a review were HF aerials I had mixed feelings, to be honest with you. On the one hand, I was looking forward to trying them out. On the other hand, the atrocious interference I suffer at my location would, I felt, render a reasonably fair review quite challenging.
The Moonraker GPA-RX Aerial
The first aerial under review – the Moonraker GPA-RX Vertical Wide Band HF Receive Antenna – is a broadband HF receiving aerial, covering a range from 2 to 90MHz. It looks very similar to a 1/2 λ CB aerial and, measuring around 5.4m in length, this would indeed put it very close to 27MHz (Fig. 1).
The aerial’s vertical radiator is mounted in an aluminium bracket and is held in place by two plastic insulators (Fig. 2). The bracket itself can be fixed to a pole using the supplied U-bolts (of 25-42mm capacity).
Alternatively, it could be fixed to a post or some other mounting by screwing or bolting through the four holes in the bracket.
On the bottom of this bracket is a small plastic box, in which a matching transformer is located.
The feeder is connected to an SO239 socket at this point.
The vertical section is a telescoping aluminium tube; each section is held in place with Jubilee clips.
I mounted the GPA-RX up at 4m in the air, with a run of RG58 cable leading to my receiving position.
For comparison, I used my 66ft Inverted L end-fed.
Both aerials were connected to a home-made switch for easy selection between the two.
I spent time using the GPA-RX both during the day and in the evenings.
Most of my tests were undertaken using my SDRplay RSP2 software-defined receiver.
On Higher Frequencies
I began on the 20m amateur band and the 22 and 19m broadcast bands. I have to add here that I find QRM on the amateur bands less severe, due to the notching of PLT devices.
However, this is not the case for other HF frequencies.
Overall, conditions never seemed to be outstanding but, during the day, there were always a large number of stations on 20m to be heard from Denmark, Switzerland, France, Germany, Italy and so forth.
Switching to my end-fed, I noted an average of 2 S-points improvement over the signals received on the Moonraker GPA-RX.
On 22m, quite the reverse was true. I could only hear the stronger stations, such as RRI and CRI, amongst others, due to the noise level. The GPA-RX was considerably better at receiving these than the end-fed.
Signal strengths were only marginally better, but there was less fading and overall signal clarity was better.
On 19m, there was not much difference between the two aerials, although PLT interference was marginally higher on the GPA-RX. I put this down to the fact that it was located nearer the main source of this rubbish.
Moving to 16m, the GPA-RX worked well with signals similar to the end-fed, although the background noise level was higher. This made weaker signals more difficult to hear.
Not surprisingly, the GPA-RX performed well on the 10m amateur bands, where the reception of signals from around Europe was typically as good – or slightly better than – those captured on my end-fed.
On Lower Frequencies
Here, I find QRM different; there is not so much of a problem (up to now) with PLT, but there is far more noise from power supplies, solar panels and my Very high bit-rate Digital Subscriber Line (VDSL).
Going lower in frequency during daytime listening, I found that my end-fed always provided better results.
It has to be remembered that it has a vertical section, which is longer than the GPX-RX and also a horizontal section, which is longer still. Because of this, I would have been surprised if my aerial had not fared better.
I did try adding a counterpoise to the GPA-RX by clipping it to the connector. However, this made no appreciable difference.
Listening during the hours of darkness on the lower frequencies was a different matter altogether, and the GPA-RX performed considerably better.
Fig. 4 is the same but with the end-fed switched in place.
As you can see, there is little difference between the two, other than that the noise floor is slightly higher on the GPA-RX.
Sometimes, the GPA-RX performed better on some frequencies and the end-fed did so on others. I even found there were instances within the same band, where one aerial would receive a particular station well but the other one would not. These variations are due to propagation conditions, under which waves were arriving at different angles, with varying polarisation and from different directions.
The angle of reception of both my end-fed and the GPA-RX will be different on different frequencies and in different directions as well, hence the reason why there were variations between the two.
I used the end-fed for the comparison, as this is a type of aerial that will be familiar to most readers.
One of the benefits of an end-fed in an Inverted L configuration, is that you get something of both worlds. The vertical section comes into its own with vertically polarised signals, and, likewise, the horizontal section favours horizontally-polarised waves.
However, space is needed to string up a wire of a decent length. For those with limited room, this may not be possible. However, those operators unable to go 'out', may well be able to go 'up'.
If that is the case, the GPA-RX could be the answer. It takes up little space and is easy to mount. The fact that it can telescope may be of use to those living in areas where it is difficult to put up aerials.
Reducing down to around 1.2m in length, it is perfectly feasible to shorten the height of the GPA-RX by lowering a couple of the sections, to make it less obtrusive when not in use.
The GPA-RX won't outperform a full-sized aerial on the lower frequencies.
However, under the right conditions, it can hold its own.
On the higher bands, especially from around 15 MHz upwards, it works well.
The current price for the GPA-RX listed the Moonraker website is £99.95.
The Moonraker X1-HF
The second aerial I tested was the Moonraker X1HF Vertical 1-50MHz Trapped Coil Receiving Antenna. This is a compact, wideband, receiving aerial, covering the range from 1 to 50MHz. It is approximately 2m long and can be mounted on a mast of up to 37mm in diameter.
The device comes in two parts (Fig. 5): The bottom part, which houses the SO239 for the feeder, is 50mm in diameter and 1m long. On top of this, is a 3/8in mounting for the whip section, which is 1.2m in length. The main body looks to be made from a sturdy white plastic tube, with the top and bottom parts held by pop rivets.
Assembly of this aerial is fairly straightforward: Screw the whip into the top of the white tube and connect a run of the feeder to the socket at the bottom.
There were no assembly instructions included with the review model, and the clamps supplied were very different from those depicted in the image of the X1HF on the Moonraker website. The web image looked as if the aerial was fixed to the mast with a pipe-clamp, whereas Shelley clamps were provided in the box.
Moonraker has advised me that the image on the website will be altered to reflect those now supplied with the X1HF. I mounted the X1HF using the same method as with the GPA-RX (Fig. 6).
Once in the air, I noted that the whole assembly was quite heavy and swayed around in the breeze. I think the majority of this weight was in the clamps.
The Aerial’s Performance
I used the same arrangement as I did with the GPA-RX (cf above). Once again, I spent a lot of time using the X1HF, both during the day and in the evenings.
I was not sure what to expect from this aerial as it looked rather too short to be efficient on the HF bands.
However, I quickly found that the results were much better than I expected.
On the higher frequencies – around the 20m amateur and 22 and 19m broadcast bands – I found that the signals received were, on average, down by two to three S-points, compared to my end-fed.
I tried the X1-HF on signals that were higher in frequency. However, some far-from-optimal conditions made an overall assessment difficult.
On the lower bands, from 1 to around 6 or 7MHz, signal levels were surprisingly good.
Like in the case of the GPA-RX, results were better in the evenings than during daylight hours. I was easily able to monitor a large number of utility stations and various broadcast stations.
Going below the quoted 1MHz, I found that reception was still possible but not ideal. However, you should bear in mind that, after all, this aerial is intended for HF operation.
Being smaller than the GPA-RX, I found it easier to move it to a location in my garden that was as far away as I could get it from the worst sources of interference. This meant adding a few more feet of coaxial cable.
I noted that the background noise dropped by a couple of S-points on the lower frequencies. Proof, if any were needed, that in circumstances where interference is a problem, aerial placement is important.
Like the GPA-RX, the X1-HF has a very small 'footprint' and, being shorter, it is even less obtrusive. It is ideal for those with limited space or legal restrictions on aerials.
It will not outperform a full-sized aerial, such as a decent doublet or Inverted L, but it may be possible to mount the X1-HF up higher than a dipole, perhaps on a chimney or a wall-mounted pole and bracket.
This will give the aerial a better chance to perform. It is quite heavy, and a decent support will be needed.
The current price of the X1-HF is £69-95.
When reading reviews on HF aerials, it needs to be taken into account that what works well in one location may not do so in another (although like everything in life, there are exceptions). The ground and surrounding objects all have an effect. Because of their design, it is possible to easily mount both aerials at various heights above the ground and in various locations. This may be beneficial when it comes to interference. Users can mount these aerials at a greater height, and as far away as possible from sources of unwanted noise.
The GPA-RX is quoted as giving excellent results with SDR receivers, and I would probably say the same about the X1-HF. Because of the lower signal output and the ability to mount it away from noise, the RF stages of the cheaper dongles will be less overloaded.
May I give my sincere thanks to Moonraker, for the very generous loan period of all four aerials they have recently supplied for review.
After chatting with a local RU reader about a new aerial analyser I have, he asked whether I would be using it to cover the adjustment and measurement of aerials. I undoubtedly will.
However, I wondered whether I could source an analyser readers would find cheap, easy to use and something to relate to, since it would be used, from time to time, in this Aerials Now column.
A search on eBay brought up a number of low-cost analysers for around £50. They might be Chinese ‘clones’ of the Sark 100. Invariably called MR100, they cover 1-60MHz and could be used on HF. Capable of measuring VSWR, capacitance, inductance and resistance, they might also be connected to a computer.
I do not know how good they are. However, if enough readers think it worthwhile, I will buy one and see how well it works. If it does perform well, I could use it as a 'standard-tool’ for my projects in future columns.
Subsequently, readers could obtain their own analysers for their measurements and tests at home, to see if these are in line with mine!
Have a look for them online and tell me what you think.
Well, that's all for this month; as always, I will reply to readers’ questions through this column.
Until then: Good Listening!
This article was featured in the November 2018 issue of Radio User