Lines of Sight, Radio Horizons and Aerial Review
This month, Keith Rawlings explains some basic ways of calculating your aerial’s ‘line of sight’
This month, Keith Rawlings explains some basic ways of calculating your aerial’s ‘line of sight’, responds to reader feedback and tests a new aerial for the civil and military airband.
Hello and welcome to this month’s Aerials Now column. I thought I would start this month off by explaining how to calculate line of sight. Clearly, the higher up the aerial is mounted, the greater the distance is, over which it can ‘see’ towards the horizon.
How is it that you can, sometimes, receive an aircraft at altitude, hundreds of miles away but you are unable to hear a ground-based ATC transmitter just a few miles away? Read on to unlock this mystery.
The calculations I use (in feet) are:
The square root of the height (H1) of aerial A multiplied by 1.412, and:
The square root of the height (H2) of aerial B Multiplied by 1.412 (Fig. 1, not to scale).
Now, let us say that TX is the transmitter site, with the aerial, at 200ft.
And say that RX is your aerial, at 30ft.
Your calculations then are as follows:
TX = 14.14 x 1.412 = 19.968 (D2).
RX = 5.477 x 1.412 = 7.73 (D1).
Therefore, the transmitter should have a radio horizon of 19.968m and your receiver aerial one of 7.73m.
Adding D1+D2 results in an overall distance of 27.698m.
VHF and UHF
In this context, do not forget that VHF/UHF signals are usually line-of-sight and a bit more. This is because there can be some signal refraction off of the Troposphere, as it curves around the Earth.
This distance can be some 15% greater than the 'visual' horizon.
When conditions are on the 'up', the distances covered will be greater.
If you have a poor feed line, bad connectors and/or high ground in between you and the transmitter, then the actual distances, over which signals can be received, will suffer.
For the VHF bands and above, the higher your aerial the better; however, bear in mind, once again, that the higher the aerial is, the longer the feeder run will be, and the greater any losses will be.
You may need to weigh up the advantages of height versus signal loss against the cost of lower-loss cable.
On HF, higher is not always better but this is something to be covered here in a future month.
To save you doing the math, you can visit this website:
Moonraker DBAB VHF/UHF Aerial
Once again, my thanks to Moonraker for another aerial to review. The Moonraker DBAB is a compact vertical aerial, which has been optimised for civil and military air band use between 108-140 and 230-391MHz. It is quoted as being a 1/4λ vertical and is 65cm in length.
The aerial has four radials, each 24cm long, which screw into a base/hub assembly. It is fitted with a decent-looking N-Type socket (Fig. 2).
Due to my interest in aviation, I was especially keen to put this aerial through its paces. As well as having a couple of discone aerials for general listening (one attic-mounted and the other mounted outside) I also have a coupled resonator, which has been cut for spot frequencies in both the civilian and military air bands.
The DBAB consists of a main vertical element, four radials, a short aluminium mounting tube that the assembly fits into, and two extruded aluminium brackets with U-bolts to attach the whole assembly to a mast.
Like the ADS-B aerial I reviewed last month, the DBAB seems to be very well made. The hub has been turned out of solid aluminium and the radials are sturdy and look like they are made from stainless steel.
Assembly is a simple matter of screwing in the radials and tightening the locking nuts.
Once this has been done, a coaxial downlead cable can be fed through the mounting tube and screwed into the N-Type socket. Afterwards, the aerial itself can be placed into the mounting tube and the small slotted screw fitted to keep it firmly in place.
The two extrusions are then placed onto the mounting tube and locked into place with further screws.
It is then just a matter to place the assembly over your mast and lock the U-bolts.
One problem I found here was that my temporary mast, which I was using to evaluate the aerial (an old Jaybeam portable mast dating from the mid-1970s) is made of one-inch diameter tubing.
The U-bolts, however, needed a larger diameter to clamp on to.
I worked around this by taking a bungee-strap out of the back of my car and used that instead (Fig. 3).
Therefore, bear in mind that, if you decided to buy a DBAB, a one-inch mast will be too small.
The Aerial in Use
I connected a 30ft length of RG58 coaxial cable, which, I confess, was less than ideal for the job. Nevertheless, I got the DBAB up in the air.
My first test was on the civil air band, where I made comparisons with my discone. Bearing in mind the discone was about ten feet higher up than the DBAB, the results were surprisingly good.
For example, Stansted ATIS was S7 on the Discone and S8+ on the DBAB. The tower frequency was the same on both aerials, and Stansted Ground came in at about one S-point better on the DBAB.
Scanning other frequencies relatively local to me demonstrated that the DBAB performed very well.
Another example was Wattisham Approach, some 25 miles away. It came in consistently at S6, as against S5 on the discone. As expected, traffic was easily heard from all over East Anglia as well as from London.
On UHF, the results were equally as good. I was particularly impressed at the quality of the signals picked up from traffic playing out over The Wash and off the north Norfolk coast.
While reviewing the DBAB, I used it to monitor the practice flights for the RAF 100th Anniversary Flypast over East Anglia.
Prior to this, I had monitored traffic involved in the arrival of the first batch of F35 aircraft into RAF Marham.
Here, a comparison with my coupled resonator (CR) demonstrated that the DBAB was down on both VHF and UHF signals.
However, it must be taken into account that the CR is higher and fed with a cable of a better specification than the one I used with the DBAB. Furthermore, the CR has been carefully tuned for best results on certain spot frequencies.
Therefore, my finding that the difference between the two aerials was relatively small, speaks well for the DBAB, which is a broadband aerial.
One thing I noted on VHF was that the levels of interference from nearby LED lighting were markedly lower on the DBAB when compared to my other aerials. I cannot explain why this might be; it is certainly not due to poor reception on the part of the DBAB. The distance from one particular 'jammer' is over 100m away, and levels on my discone can be around S7-S8. However, on the DBAB the level was around S4.
I ventured outside of the air bands and found that, on VHF, the DBAB worked well up, to the limit of my tests at 180MHz. Signals were typically two S-points down, compared to the discone.
On the Broadcast FM band, the results were similar. From the range of 400-470 MHz onwards, the DBAB was considerably down on the discone but could still be put to good use on the stronger/local signals.
One has to always remember that this is actually a dedicated airband vertical.
I took measurements with a DG8SAQ Vector Network Analyser. The aerial was 15ft above ground, and calibration was made at the aerial end of a short run of RG213 coaxial feeder cable.
I measured the Voltage Standing Wave Ratio (VSWR) between 108 and 136MHz.
At no point did it rise above 3:1; the lowest reading was 1.4:1 at 132MHz.
On UHF overall, the readings were not quite as good. The lowest reading was 1.45:1 at 236MHz and 2.7:1 at 337MHz. The image in Fig. 4 represents a sweep, from 100 to 450MHz, showing VSWR on the yellow trace, with the reference position being the baseline.
You can clearly see the readings between markers one and two, which span the range of 108-137MHz.
Markers three and four bracket the span between 230 and 391MHz, with marker five highlighting the highest reading, at roughly mid-band.
The graph in Fig. 5 (blue trace) shows the resistance. Again, markers one and two span the range of 108-127 MHz, with markers three and four ranging over 230-391MHz.
Finally, markers five and six show high spots on the trace.
The 50Ω reference position is two lines up from the base, as marked on the right of the image, with a resolution of 50Ω per division.
Details of the markers can be seen in the centre of both images.
Overall, I was impressed with the DBAB aerial. It performed well, especially considering its size. This is noticeable because it is shorter than what may be expected from a VHF air band vertical aerial.
The aerial has a low visual impact, is simple to assemble and comes with a reasonable price tag. It yielded great results, at less than ideal height, and with a mediocre run of cable.
It also gave useful service outside of the air bands for the casual listener.
If your interest is in aviation, this aerial could be ideal because, all in all, it is a great little performer.
The DBAB presently retails at £44.99.
I received an interesting email from Ian, who lives near Bristol. In part of it, he explained how he works as a shop fitter and some time ago, when undertaking a job in a retail park, he noticed workmen taking down some aerials. The one Ian saw had a number of 'loop' aerials on it, which "looked considerably larger on the ground than it did in the air". According to Ian, it also "looked to be very 'tatty'”.
Ian explained to the workmen that he was a radio enthusiast and asked if they were scrapping it and, if so, could he ‘take it off their hands’? He was informed that it would be of little use to him because "there would be nothing he could now listen to on it" and anyway "it was a cardioid antenna, so he wouldn't be able to do much with it". Therefore, Ian left it and now asks me if I think the aerials would have been of use.
From Ian's description, I am guessing that the 'loop' aerials were UHF folded dipoles mounted to a pole. As the structure was located in a retail park, I am also guessing that it may have been part of something like Shop Watch. The operating frequency would likely have been in the region around 450.0MHz.
That he was told there would be "nothing he could now listen to" maybe hints at the possibility that the new aerials might have been for a new digital system on that site.
It is often thought that digital systems cannot be listened to, except by users registered on that system. The workers’ comments could also have been a hint that the system had been encrypted.
From what the engineers stated, my assumption is that the dipoles were configured to form a cardioid pattern (Fig. 6). This image was generated from a standard model in EZNEC.
Note the power radiated in the forward direction and the deep null at the back.
The array may have been set to 'beam' over the retail park so that maximum energy would go where it was wanted. It could also be that the null at the back was used to reduce interference to (or from) other locations –
or all of the above!
Incidentally, there was once a land mobile base station near to the coast that had a cardioid array installed to concentrate signals over the land. It worked very poorly, except for mobiles driving between it and the sea.
Eventually, it was discovered that it had been configured incorrectly and was happily beaming signals way out over the North Sea and in the direction of Norway!
Back to Ian: As long as the aerials were not too corroded, I think they may have been of use. They could have been taken individually or re-instated as a cardioid system. This may have been convenient for direction-finding purposes, by using the null off the back to pinpoint the direction of a signal.
Lastly, John Horobin asks if I will be describing how to work out the length of an aerial for a particular frequency.
The answer is yes.
There are, however, a lot of formulae for many different aerial designs. Therefore, I will do this consecutively, as we progress through different aerial types over the coming months.
That's all for now; as always, I will reply to readers’ questions through this column.
Until then: Good Listening!
This article was featured in the October 2018 issue of Radio User