VHF Antennas on kingposts

[Dieselten]

If most aircraft transmit vertically-polarised signals on the comms radio (i.e. electric field propagates in the vertical-plane) and you mount your antenna horizontally (which will receive the electric field component propagating in the horizontal-plane) then you will get several dB signal loss, depending on the angle betweeen the receive antenna and the plane of the electric field. In theory, that is.

 

The name of the game is to maximise the transmitted field-strength in the primary lobe of the antenna radiation-pattern, and maximise the number of microvolts at the antenna terminal on receive. A resonant antenna, impedance-matched to the transmission-line and mounted in the clear, free of shielding by adjacent structures, will achieve both, all other things being equal.

 

If the antenna for the transmitter and that for the receiver are at exactly ninety degrees to each other then the signal loss may exceed 30dB (ten raised to the third power), but this in practice is rarely achieved for anything other than very brief time periods due to the movement of aircraft around the fore-and-aft axis. However, to maximise received signal strength it is prudent to mount your comms radio antenna so it is vertically-polarised (i.e. the whip is vertical when in level flight). Ground stations transmit aircraft comms vertically-polarised, as do most aircraft.

 

It should be noted VOR transmssions are horizontally-polarised which explains why VOR receive antennas on aircraft tend to be folded whips or blade antennas mounted horzontally, often in the vertical fin. If you have a combined COM/NAV radio, or are using a second radio for VOR reception, then the antenna for the NAV receiver should be mounted horizontally.

 

The V-Rabbit antenna is a very good solution and works extremely well for the comms radio. I consider it to be well worth the investment.

 

Having become very browned off with striking the tip of my kingpost-mounted radio antenna on the hangar door, I have recently moved it to a small stub of tubing mounted where the sting for the Cruze wing normally resides. I'll put up with any slight shielding by the kingpost for the convenience. In practice there will be little discernable difference in radiation-pattern anyway. The underside of the antenna is a null-point, so this will minimise any potential ignition-system noise.

 

Coaxial cables are lossy; use the shortest practicable length and use the highest quality cable you can afford, properly-terminated with good quality connectors. An "air-gap" can cause major attenuation of both transmitted and received signal.

 

 

[Guest davidh10]

I'll second everything Diesel has said, although I haven't tried a "V" antenna.

 

Another reason you don't usually see a 30db loss in signal strength for cross polarised antennas in practice is that polarisation is changed by reflection from hills and man made objects. Thus the received signal polarisation is seldom all in one exact plane and is often received at different times due to reflected path length being longer than direct path length. That also contributes to varying signal quality.

 

Remember analogue TVs and ghosting! Also remember the patterns of ghosting you saw when a plane flew between your antenna and the transmitter, creating an added reflected signal that varied in phase to the direct one. It's just easier to see the effect on a TV than on the radio, but the same effects occur.

 

You won't notice it with local transmissions, but where it counts, at distance, it can be the difference between hearing / being heard and not.

 

 

[Dieselten]

Test-flew the re-located antenna this morning for 2 hours at 4500 feet and found no impairment to reception of YWOL CTAF (127.3), Sydney Radar (124.55), Nowra Approach (123.5), Melbourne Centre (121.2), Sydney Approach (128.3), Sydney Departures (129.7), and Nowra Tower (118.85). YWOL AWIS (120.35) also boomed in, although it can be a little unreliable below 4500 feet west of Moss Vale.

 

When you are orbiting at 4500 feet you can do a lot of eavesdropping!

 

As this spread covers 11Mhz of bandwidth and includes all the frequencies we might reasonably be required to use, I conclude that mounting your radio antenna at the rear of the wing (on the sting or on a stub in place of it) is operationally as efficient as mounting it at the top of the kingpost, with the added benefit of reducing the height-clearance issues for getting the rigged trike into the hangar.

 

No ignition noise breaking through the squelch was detected using normal squelch settings (as if the antenna were still mounted on the kingpost).

 

"Gentlemen, mount your antennas!"

 

 

[cscotthendry]

"Gentlemen, mount your antennas!"

I now have three trikes using my mount. The antenna seems to work well in this position. So well in fact that Airborne are designing a mount. I angled mine to give a bit extra prop clearance when the bar is fully forward. Shane Duncan said that you should have at least 100mm clearance between the antenna and the prop. It wasn't clear why, but I got the impression it may have to do with regulatory issues.

 

The other day, I was flying at Teewah, north of Noosa and I was hearing calls (5/5) out of Tyagarah. I was at 2500' AMSL at the time.

 

 

 

[alf jessup]

I now have three trikes using my mount. The antenna seems to work well in this position. So well in fact that Airborne are designing a mount. I angled mine to give a bit extra prop clearance when the bar is fully forward.

Geez Scott,

 

Looks mighty similar to mine lol

[cscotthendry]

Alf:

 

That's because I got the idea from you. Credit where it's due. I used a piece of angle to mount the antenna where I think you used the Airborne bracket or similar. Also, my bracket mounts on the end of the plug where yours mounts on top. Six of one, half dozen of the other. I started out without the angle, but like you, didn't like how close to the prop the bottom was so I used the 10 degrees that you mentioned and that was good. I had a look at the antenna in flight and the bottom doesn't flex in the wind at all, so maybe the angle is not really necessary.

 

 

[alf jessup]

Alf:That's because I got the idea from you. Credit where it's due. I used a piece of angle to mount the antenna where I think you used the Airborne bracket or similar. Also, my bracket mounts on the end of the plug where yours mounts on top. Six of one, half dozen of the other. I started out without the angle, but like you, didn't like how close to the prop the bottom was so I used the 10 degrees that you mentioned and that was good. I had a look at the antenna in flight and the bottom doesn't flex in the wind at all, so maybe the angle is not really necessary.

Scott,

 

It was said with tongue in cheek mate tongue in cheek.

 

I wanted no credit mate if I can help others out with a problem i will always put my foot forward as I guess you would be the same.

 

I had a chuckle when i wrote it.

 

Was a pleasure to be able to give you and idea mate.

 

Alf

 

 

[cscotthendry]

Alf:

 

Yep, I got the humour, but still...credit where it's due. I had been thinking about the antenna issue for some time and tried it on the base, but that wasn't much chop. Since I have a lathe and mill, your idea looked very do-able and it also works well electronically. Good on 'yer.

 

 

[eightyknots]

P.S, mind you, all that theory aside, our radios at 5 to 6w of power are grossly overpowered for what is needed for line of sight work. That power can produce issues (see ferite discussion) but can also wallpaper over alot of other issues such as long coax run and inline connectors etc. If it works, that is, your clear in the air, you can hear people who are operating close to you and there arent any other reasons to poke about then in general...dont.

It is not so much the output power: as Andy pointed out the 5 or 6 Watts tends to cover any minor shortcomings in the coax feed or the way the antenna is mounted. What is really compromised is the radio's receiving ability. I may only respond to a signal of around one microvolt. If you have too many unnecessary connectors or an antenna mounted in a very sub-optimal way, you may not receive the signals you would otherwise receive.

 

 

[Dieselten]

Technically for AM radio receivers we should refer to a sensitivity of so many microvolts at the antenna input for so many dB signal-to-noise.

 

My radio training dates from 1980 and is therefore a little dated, but I seem to recall the input sensitivity of AM broadcast receivers were usually rated at a few microvolts (say 2-5uV) at the antenna input for 20dB signal-to-noise.

 

A single sideband receiver was a more sensitive beast, down to below a microvolt or less if the front-end was really hot or used a special low-noise amplifier, which specialised communications receivers did (the 40673 dual-gate IGFET springs to mind). These sensitivity figures relate to superheterodyne receivers, but I think much the same applied for tuned radio frequency receivers on the AM broadcast band - somewhere between 2-5 uV for 20Db would be fairly normal. However, the AM broadcast band is Medium Frequency. We need to be careful lest we try to compare apples to oranges.

 

Aircraft radios are all superheterodyne receivers, and probably double-conversion ones at that with a first IF higher than the received frequency for better image-rejection. (The second IF is most likely the ubiquitous 455Khz.) The transmissions take place in the high-band VHF portion of the radio spectrum, and the bandwidth of the transmissions is necessarily narrower than AM broadcast (we only need "communications quality" audio on aircraft radios), but I still think the sort of sensitivity at the front end would be still be a couple of microvolts for 20dB signal-to-noise. Basically the front-ends of aviation-band radios are designed for the field-strengths typical of radio waves propagating through free-space, unimpeded by ground or absorption by surrounding landforms etc.

 

We also need to bear in mind the operational characteristics of our radio use; isolated and intermittent periods of transmission at relatively low power across a range of frequencies. Generally, our runs of coaxial cable are relatively short (a couple of metres is about the longest run of coax you'd ever need on a trike), and the loss in dB per metre is relatively small. However, the loss in dB is the same on receive as it is on transmit.

 

We require a radio system which has enough radiated field-strength from the transmitter and antenna (the transmit system) to enable us to communicate over a range suitable for the characteristics of the aircraft, and receive system performance to clearly receive transmissions from aircraft within a reasonable distance of our own for safety-related or other operational requirements. In the process of rigging our cables we have to use a certain number of connectors, and a length of cable. We also have to choose where to mount both the radio itself as well as the antenna. Invariably we compromise, but compromise is one thing; using cheap and poor-quality cable or lossy connectors is quite another.

 

In an ideal world, the top of the kingpost really is the best place for the antenna. But the world of trike-flying is far from ideal. The best compromise mounting-position we can achieve that satisfies the requirements for transmit and receive performance is more than adequate. If that mounting position allows for convenient manoeuvring in and out of hangars, or easy access for installation/removal of the antenna when rigging/de-rigging, then we have a workable solution.

 

I think we have enough evidence from the recent posts to conclude that mounting on the sting, either vertically upwards or in an inverted position, will suffice perfectly adequately, provided there is no de-sensing of the receiver due to nearby electrical fields emanating from engine electrical systems.

 

 

[cscotthendry]

In an ideal world, the top of the kingpost really is the best place for the antenna.

Sorry, but I just have to disagree with this point. It's what started this whole thread and the effort to move the antennas around. Here's why.

 

The most common antenna supplied with Airborne trikes is a counterpoise type. Even though the counterpoise does not radiate signal, it's electrical characteristics are tuned for the antenna. If the antenna is mounted on the kingpost with the active end pointing down as supplied by Airborne, the kingpost loads and shadows the antenna radiation pattern. This also increases the VSWR which is not good for the transmitter outputs and could even be fatal. If the antenna is turned up the other way, with the active end pointing up, then the counterpoise lies alongside the kingpost. This changes the effective RF characteristics of the counterpoise and consequently the balance of the antenna. Again, not an ideal situation.

 

I don't really know how the V rabbit type antennas work, so the top of the kingpost may be ideal for them, but it's defininately a no-go zone for the counterpoise types. Nor is it a good place for groundplane type antennas.

 

Like Dieselten, my RF knowledge is very rusty (USAF C.1970's; NASA tracking station C. 1980's) But the good people at Mobile One (who manufacture the antennas supplied by Airborne) confirm that the kingpost mount is a no-no.

 

BTW, This post wasn't meant to be a flame, and I hope it didn't come across like that.

 

 

[Mc Guyver]

Sorry, but I just have to disagree with this point. It's what started this whole thread and the effort to move the antennas around. Here's why.The most common antenna supplied with Airborne trikes is a counterpoise type. Even though the counterpoise does not radiate signal, it's electrical characteristics are tuned for the antenna. If the antenna is mounted on the kingpost with the active end pointing down as supplied by Airborne, the kingpost loads and shadows the antenna radiation pattern. This also increases the VSWR which is not good for the transmitter outputs and could even be fatal. If the antenna is turned up the other way, with the active end pointing up, then the counterpoise lies alongside the kingpost. This changes the effective RF characteristics of the counterpoise and consequently the balance of the antenna. Again, not an ideal situation.

 

I don't really know how the V rabbit type antennas work, so the top of the kingpost may be ideal for them, but it's defininately a no-go zone for the counterpoise types. Nor is it a good place for groundplane type antennas.

 

Like Dieselten, my RF knowledge is very rusty (USAF C.1970's; NASA tracking station C. 1980's) But the good people at Mobile One (who manufacture the antennas supplied by Airborne) confirm that the kingpost mount is a no-no.

 

BTW, This post wasn't meant to be a flame, and I hope it didn't come across like that.

Hi Scott,

The Vee Rabbit is designed to mount on the king post but below the peak. That way it does not get damaged when moving in and out the hangar. I fly an SST and I have the aerial mounted horizontally on a short keel extension so that I can get it in and out of my low hangar without causing damage. I have an extremely clear transmission and reception well over 100KMs. (do we need better in a trike?) This with a hand held Vertex I have yet to have people say I am unreadable in normal circumstances in fact people remark how clear my transmissions are:clap:

 

 

[YFT]

Hi All, I am not going to buy into your debate about which is the best aerial. The one in the first post is a Dipole aerial, and in the photo it is mounted up side down. I have used that aerial for over 20 years on trikes. I also have one of the best radio responces from airservices and other aircraft. You need to have a full system, that means radio, aerial, and comms. get it right it works, get it wrong it give you a problem. If you don't believe me as David, or, Glen or anyone that has been on a Megafauna Flyaway, and they will let you know about my radio system.

 

All the best

 

Peter.

 

 

[pudestcon]

Hi All, I am not going to buy into your debate about which is the best aerial. The one in the first post is a Dipole aerial, and in the photo it is mounted up side down. I have used that aerial for over 20 years on trikes. I also have one of the best radio responces from airservices and other aircraft. You need to have a full system, that means radio, aerial, and comms. get it right it works, get it wrong it give you a problem. If you don't believe me as David, or, Glen or anyone that has been on a Megafauna Flyaway, and they will let you know about my radio system.All the best

Peter.

G'day YFT,

 

How about you tell us about your system? We could all benefit from knowing what is so good about it.

 

Thanks in advance.

 

Pud