VHF Antennas on kingposts

[cscotthendry]

Hi all:

 

When I purchased my new trike I ordered it equipped with a VHF comms radio. The manufacturer mounted the antenna on the kingpost as in the picture.

 

On a recent trip, I was told in no uncertain terms that the mounting was WRONG. Not being the type to just take the word of armchair experts, I asked some experts in the antenna and radio business, including the manufacturer of the antenna on my trike. It turns out that all agree that this method of mounting is as wrong as you can possibly mount an antenna.

 

The problem is this: The antenna has a stiff side and a flexible side. The stiff side is the active side and the flexible side is the "counterpoise" or inactive side. The way my antenna was mounted, the active side extended down alongside and very close to the metal kingpost. Having a metal object so close to the active side of the antenna severely hinders the transmission signal and adversely affects the VSWR between the transciever and the antenna. Worst case, this can cause the output drivers of your radio to be burnt out.

 

The engineer at Mobile One (who manufacture my antenna) said that he would not reccommend mounting such an antenna on a metal kingpost under any circumstances, and most definitely not the way mine was mounted. He said that if the antenna must be mounted on the kingpost, it should be mounted such that the active end is at least 300mm away from the metal, measured in the horizontal plane.

 

If your antenna is mounted like this, it would be best for your radio and your transmission capability if you move it.

 

 

 

[Guest Andys@coffs]

So as an ex RAAF Radio Tech a couple of points that are worth considering:-

 

1) Mounting on the kingpost means that the coax cable run is probably at least 7m in length (longer is bad)

 

2) You'll need a connector on that run somewhere so you can dismount the wing (Connectors are bad)

 

3) The antenna you have is a 1/2 wave dipole, 1/4 is active as you've said and the other 1/4 is the rubber ducky antenna.

 

4) Of a std stainless steel 1/4wave antenna and a 1/4 wave rubber ducky antenna you would only use the latter as the active element for transmission when there are factors that drive the need for short stubby and flexible

 

5) If I had to mount this on the kingpost, Id be inclined to flip it 180degrees so the stainless steel is upright (and above and clear of the kingpost), but that then exacerbates the height issue and hangar door problem that trikes have. It is for that reason alone that I suggest the rubber ducky is currently upright and mounted where it is.

 

So, all that said, on my trike I mounted a 1/4wave stainless steel antenna just on the outside sidet of the pod so that the trike metal support bracket onto which the pod fiberglass is fixed is used as a (poor) ground plane. In my case I achieved an SWR ratio very close to ideal despite the poor ground plane. The benefits of such a mounting are that the coax is short, has no connectors (other than the one to the radio, but every installation will have that). The drawbacks, its close to your headset so can induce feedback, I solved this with a ferrite core on each of my headset and microphone and PTT switch leads (these were commonly seen on computer monitor leads, they allow low freq signals to pass through the wire but block high freq like a VHF transmission, readily available at places like Jaycar etc). The antenna is also close to the pilot which isn't desirable, but then I don't talk much (when in the air). Also the top of the antenna is close to eye level when you reach down to lift the nose for ground handling purposes...

 

Andy

 

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.

 

 

[cscotthendry]

Andy:

 

I saw a video on YouTube of a trike in Hawaii that had the antenna mounted as you describe. I thought about trying that as well for the reasons you state. My only reservation for having it there (I hadn't thought of the ground handling issue) is interference with the covers.

 

Currently I have my antenna mounted on the trailing arm of the starboard suspension the way it is on the trikes with the topless wing. My transmissions are reportedly better, but I get some engine ignition noise on receive of weak signals.

 

I should add that I'm an ex-USAF avionics technician, but antennas were never my forte'.

 

I delved into this as I and my instructors were getting some "motorboating" type of signal breakup on the "blind side" of our radio pattern, that is the side that was shielded by the kingpost.

 

 

[Tracktop]

Hi Scott

 

I assume you read my posts in

 

http://www.recreationalflying.com/forum/avionics/71456-aircraft-vhfs-2.html

 

post 12 for example

 

So I am also looking for an answer

 

Mc Guyver uses a rabbit ear setup that I think pokes forward and up, half way up the king post. He claims they are brilliant. This may be a suitable solution but I would need to try it before recommending it.

 

Solves the height problem, and he says puts out plenty of signal

 

 

[cscotthendry]

Ray:

 

What you described in your post about having to turn around to be heard is exactly why I moved my antenna. I had the same problem between myself and my instructor on a recent flight. The reason is the radio shadow caused by the metal kingpost.

 

As for the rabbit ears, I have seen V shaped VHF antennas on some GA aircraft. I don't know how well they perform, whether they need a groundplane, and whether they would be suitable on the top of a kingpost installation.

 

 

[skeptic36]

Hi guys,

 

Just dug out the HGFA Pilot Training Work Book,

 

Question 12.20: Where is it best to fit a VHF antenna a weightshift microlight?

 

Answer: On top of the Kingpost.

 

So I guess they better have a re-think on that one :ne_nau:

 

 

 

Regards Bill

 

 

 

[cscotthendry]

SHOCK!

 

HGFA documentation with out of date or wrong information?!?! NEVER!:bad computer:

 

 

[eastmeg2]

Hi Guys,

 

I've decided to try mounting the aerial on the keel extension of my Buzzard. Depending on how it goes I might do the same for the XT later on, though I have some reservations about whether the top wires might reduce the amount of signal going forwards.

 

I've yet to buy the coax cable and mount the radio. Maybe I'll try it on the XT first since it already has a radio and coax to the hang point.

 

We'll see . . . :ne_nau:

 

Cheers,

 

Glen

 

 

 

 

 

[Tracktop]

Certainly worth a try. Look forward to the results.

 

 

[skeptic36]

Hi Guys,I've decided to try mounting the aerial on the keel extension of my Buzzard. Depending on how it goes I might do the same for the XT later on, though I have some reservations about whether the top wires might reduce the amount of signal going forwards.

 

I've yet to buy the coax cable and mount the radio. Maybe I'll try it on the XT first since it already has a radio and coax to the hang point.

 

We'll see . . . :ne_nau:

 

Cheers,

 

Glen

Hi Glenn,

Having read Andys' post, I think we need to get off the wing completely as that will eliminate the need for 1 joint in the coax and a whole heap of length in the cable.

 

Another problem I see with mounting it on the keel would be that it will be in your way when you are pulling the tension on the wing, although if it has a screw off type of whip on it (which it will have to be if you want to bag the wing) I guess that wont be to much of a problem other than one more thing to do.

 

Regards Bill.

 

P.S sorry mate I don't mean to be critical of your idea just constructive.

 

 

[Tracktop]

another overseas site I visit is also discussing similar things - no not my doing..

 

A suggestion there was to mount it underneath in the centre of the fuselage pointing down ( 3 axis).

 

They seemed to think that most of the work was done at the base of the aerial and that you could quite successfully bend the aerial in the middle so the whip end pointed back at about 45 - 60 deg ( I think ) thus reducing the length and stopping it dragging on the ground.

 

Now I have just written this - the same would apply to the long section mounted up from the king post, though I don't know if it would reduce the height sufficiently for most of us.

 

I am also not convinced that the additional plug required to mount the aerial on the wing is a major issue so long as it uses the correct and quality product - 50 ohm not 75 ohm connectors and the termination is done correctly.

 

Also we are transmitting EMR so the further the aerial is away from the body the better - the wing is a good place.

 

Moving it to the keel extension may make it more susceptible to engine interference.

 

 

[eastmeg2]

I was told not to buy my coax from DSE since what they sell has such an open and leaky braid that the coax will transmit (and presumably receive) almost as much as the antenna. That's an exaggeration I'm sure but the message is clear. Was also told that a well terminated BNC connector should only lose about 0.1dB of the signal, so provided that the care is taken to terminate the BNC connectors properly the losses shouldn't be noticeable.

 

 

[cscotthendry]

Hi All:

 

When discussing the antenna with Peter from Mobile One, he suggested double shielded cable would be best for the coax run. He didn't name any brands though.

 

Also, be aware that there is RG58U and RG58CU. RG58U has a solid inner conductor, CU has a stranded inner. Maybe Andy can shed some light on whether one is better than the other.

 

With any coax you buy, strip a bit of insulation off it in the store. If it's the type with the foil sheild and very few copper strands for the shield, don't buy it. If it has a braided shield, it's the better quality stuff.

 

 

[Guest Andys@coffs]

At the low frequence we are dealing with whether the core is solid or multistrand doesnt change the electrical characteristics too much. Its more about flexibility. Multistrand is able to cope with occasional bending much better than single core. At higher frequencies than we use transmission occurs on the outside of the central conductor. In that case multicore has more outer surface area and is generally a better conductor (all other characteristics being the same) up to the point that the frequency gets so high that wavegiudes are a better option (RADAR frequencies etc).

 

Regarding insertion losses due to connectors, yes they can be low if the connectors are good quality and you have quality and correct crimpers for the job....most people dont. Also insertion losses, as I mentioned in my last post, arent that much of an issue on tx due to the power levels we have, but are an issue for rx. Any db loss is important especially when people understand that db's are a logarithmic scale and a small number doesnt necessarily equate to a small impact. to put that into context for people 3db is a doubling in power so a 9db reduction represents 7/8th 's loss. Now I know that the insertion loss you identified is nowhere near that, however the reason we are having this whole discussion was due to situations of poor rx...any insertion loss is significantly detrimental to that cause. Aslo insertion losses are incremental, that is there is one for each connector and I have seen a number of installations using BNC where there is a male BNC on each side of the wing and a gender changer in the middle so 3 lots of insertion loss where in my case there is none.

 

Dollars are generally the driving factor here. Quality coax and tools and connectors are quite a bit more expensive than could otherwise be had, however when you consider that most will have paid between $1k and $2k to put in a radio and intercom saving $10 doesnt really make that much sense when it makes the other expenditure less effective. Of course the longer the coax run the more important its quality is.

 

Im still not convinced that the top of the kingpost is the best place. Where I have mine mounted has never caused me any issues other than those I mentioned earlier around ground handling (in my instal) and Ive never had any issues of tx or rx clarity or reach where I wasnt convinced the problem was with the B end. Its about as far away from the engine and its sources of noise as I would get with the kingpost with none of the dissadvantages of the kingpost (long run and insertion losses)

 

 

[Tracktop]

Hi Andy

 

Whats your thoughts on the Keel post location? discarding the fact that you would rather not have it on the wing?

 

Also had a suggestion ( indirectly from Microair) that some of our close proximity comms issues could be "blasting" where the power setting on the radio is set for long rather than short distance comms. Thoughts on that? At present I tend to think is is still shadowing as had lots of trouble communicating with another trike in company flying on Sat. Both are set up exactly the same ( x factory ). Communication varied depending on trikes location at similar distances 200m- 500m.

 

 

[Guest Andys@coffs]

Ray

 

In an ideal world the best 1/2 wave center fed dipole sits in splendid isolation from all other things, with the coax run being as short as possible and leaving at exactly 90 degrees to the antenna plane.

 

Of course real life is not like that and as such you have to be aware of a few aspects to antenna design and consider them in your context.

 

The old Yagi antenna that most have seen as a form of TV antenna is a special folded dipole with a bunch of additional metal at strategic locations. That other metal takes the dipole radiation pattern, which is essentially like a donut in shape around the antenna, max power out at the 90degree to the antenna element and minimum output looking down the antenna element, In our case directly above and below us, and changes it to look closer to a spotlight where maximum power is one single direction and very little in others. In this case it was just the location, shape and length of those other bits of metal that caused that. As such any metal (or conductor, and yes that can include the coax) in close proximity to the antenna will change the radiation pattern.

 

For our use we want the radiation pattern to be like the donut. We want good tx and Rx in all directions equally. Like a good donut we want it to be as fat as possible so that altitude has minimal impact and we don't have issues txing and rxing to Airfields/planes that are steep angles below us, ie close to the poorest radiation location directly beneath us.

 

So, no metal in close proximity....hmm that cuts out most aircraft so we have to compromise. Metal that is at 90 degrees to the antenna, such as the wing leading edge (for the example of a kingpost mounted antenna), will have little impact (No I didn't say no impact, just less) than metal that is parallel to the antenna plane (Think Kingpost itself ). Where metal is parallel to the antenna then the distance between the antenna and the metal has the major impact on what effect it will ultimately have. For those that are interested goggle 1/4 wave traps. These are used in a number of locations like Microwave oven doors etc to ensure that no radiation escapes even if there is an air gap that might otherwise have you worried...

 

So, to the question, kingpost location. I don't like it and cant see how you can mount an antenna so that the resultant pattern will allow good all round Tx and Rx. Is my alternate position any better, experience says yes, however there is the front bar that is close to running in parallel and its about a 1/4 wavelength away so it will have some impact on the pattern. On the main undercarriage? No not for me, too close to the engine where we have moving metal. Moving metal creates noise as does the ignition and electrical system. and so what does that leave us with...well not much really and thats why the majority of antennas are on the kingpost not because its good, but because there aren't many alternatives.

 

Regarding "blasting" no thats not an issue. On the ground towers and aircraft are always in close proximity and they don't have any issues. Modern radio design is like those transition lens in sunnies, except more instantaneous, if its too loud (or bright) they just adjust to suit, if its 100's of miles away the attenuation is removed so it can be heard. The microair radio is perfectly capable of working without any issues at all in a trike and if you do have issues its almost certainly to do with the antenna or intercom system

 

Andy

 

 

[cscotthendry]

Hi All:

 

If anyone is interested in experimenting with this, I have a proposal:

 

I have a lathe and milling machine and I'm not afraid to use them.

 

I would like to try mounting my antenna on the front of the pod, but I don't want to drill holes in the fibreglass. Also, I'd like to have a mount that doesn't get in the way of the covers, or require the covers to be modified.

 

I have an idea for this, but any others are most welcome.

 

On my base, the front fork is sealed by a piece of soft neoprene (like wetsuit material). This is attached to the fibreglass by a set of screws. I thought of making a mount that would attach by these screws, with some kind of quick removal facility or the ability to rotate the antenna 90 degrees for ground handling. A quick removal mount would have to have some kind of attachment pin, preferrably something readily available and cheap (which rules out pip pins). A rotating mount would have to have some kind of spring latching mechanism.

 

Anyone interested in getting involved to help with a design for something like this?

 

 

[Guest Cloud Warrior]

Really glad this topic is active at the moment as I am getting increasingly frustrated with my current comms setup's performance. I use a Black Widow TT2000 fitted to the rear of my kingpost (as per manufacturer's instructions) connected to an Icom A5 hand held radio. I can hear other aircraft operating in and outside the circuit up to about 3 miles away and after that nothing. They don't hear me when I transmit either. The transmitting aircraft also have to be behind me for me to hear them. I presume therefore that the kingpost is masking any signals received from stations transmitting in front of me as well as my own transmissions.I am sure the radio is working fine as we taped some of my transmissions from a radio on the ground while I was in the circuit and I have also used it as a handheld around the airfield fitted with a rubber ducky antenna. The culprit has to be the antenna and its location.

 

So I am looking at getting a Vee Rabbit antenna from South Africa which can then be fitted to the kingpost again (all my old flying colleagues from SA swear by them) or going back to a wire antenna mounted in front of my trike as per the original arrangement when I bought the trike. My comms were definately better when I had the antenna set up like that. Only problem I had with it was I nearly poked my eye out on a number of occassions when moving the trike hence my decision to fit a suitable antenna to the kingpost.

 

 

[Guest Andys@coffs]

The 1/4wave dipole I used, which came from Dick Smith (But ultimately was manufactured by Mobile One) had a simple screw on thread. It was easy to remove the antenna from the mount if needed. That left just the bracket on the outside and it had no sharp corners at all. All the mobile one antennas are here Airband Antennas look for the cheapest one...that was me. Note the comment that the antenna isnt tuned and needs to be cut to length, against the chart thats included with the antenna. As I recall I just cut it for middle of the voice component of the aircomms band (No not middle of the Air band cause 1/2 of it is for VOR use only). If you dont cut it to length its going to have terrible performance because its too long and therefore is tuned for a frequency lower than the voice airband. Alternatively you could add a spring in line (and cut the same length off the whip) and just hook it down to move it and avoid a poke in the eye...that said a bent down spring is an invitation to a whipping...yours!

 

I note that there is a fibreglass one with a BNC connector, Cant get easier than that to take on and off (pressuming there is an equivalent BNC base on which to mount it.

 

You will need to check the resulting output with an SWR meter that covers the airband. This is the first test that proves everything is OK to this point, the rest of the testing is just airborne Rx and Tx performance. The SWR test proves that you arent going to damage anything by trying to Tx into an open circuit, or an unbalanced load.

 

As I recall the base, antenna including cable and connector was about $50 back a few years ago, but it works well. That said I suspect that the centre fed 1/2wave dipole mounted in the same location will also work just as well... The downward rubber ducky is bound to get a comment or two due ugliness (at least I think its ugly) but hey, it'll probably work.

 

Andy

 

 

[Tracktop]

getting a Vee Rabbit antenna from South Africa which can then be fitted to the kingpost again (all my old flying colleagues from SA swear by them)

That sounds the same as the one Mc Guyver swears by.

 

 

[Tracktop]

Just did a bit of a search on them and found this site that makes interesting reading

 

6 antennas on trial [Archive] - Microlight Forum

 

Note however the manufacturers comment near the end

 

"Its not advisable to use a radio with more than 3 watts carrier as the output will crossover on to other frequencies as they are designed primarily for hand helds and precisely as you used them."

 

mmmmm ????

 

Here is another link with pics about half way down

 

Air Magic Products

 

 

[Guest Andys@coffs]

Ray

 

Ive got to say that a SWR of 1.9 is not that good. Most manufacturers of Antenna's specify that it must be less than 1.5 over the entire freqency range. Anyone who knows antennas will only be happy when the SWR is around 1.1 when its that or lower you know that the majority of your power is being radiated into space and not returned as a reflection to the output circuit of the TX portion of your radio.

 

I also note that the testing was on a handheld, whereas the majority (i Belive) in Australia are using the Microair or XCOM which are both likely better radios than the handhelds. Where you aware that the design engineer for the Microair and the Xcom was actually the same guy, M first then X.... makes me laugh when I hear Xcom or Microair owners slagging off the other brand.... That said a bit of googling might show that the entrepeneur(s) behind each might well have very different business ethics from each other...

 

I also read the last part of the post and to be honest I think the guy that made that post was long on marketing and short on engineering. There are a couple of things in that post that make my BS detector go off. Close transmissions.... what a load of waffle, radios are very good at attenuating powerful signals. The analogy is a piece of wood, very easy to make it smaller, much more difficult to make it larger. That is, Radios can easily attenuate a signal but cant make one appear where the antenna doesnt capture it. A modern radio is most unlikely to be overdriven.

 

Crossover to other frequencies......total BS an Antenna is a passive device it cant change its characteristics as a result of power (other than lots of power may melt it etc...in that case its characteristics will change...lots of power doesnt include 5-6W, add 2-4 zeros on the end and thats a different story)

 

Different frequncy centers for different localities, probably another load of BS as the differenc ebetween the centre from highest to lowest was just 3mhz. At a SWR of less than 1.5 3Mhz on 120Mhz, where the antenna actually operates from 118 to 136Mhz is unlikely to be real. Technically it could be done but the likely impact would be fine tuning that last 1%of performance. and really 126.79(Au) vs 127(Uk) bollocks!, the difference is likely to be sub mm in length change.

 

Antena location, end of the keel pointing upward.... yeah I can see that working, but dont like the closeness to the engine. Providing its not sitting at a multiple of 1/4wavelength away from the kingpost then probably not a bad location. Thinking aloud, front of the keel post pointing down near the absolute front of the wing, perhaps poking through the undersurface skin zip, again, as long as its not at a multiple of 1/4wavelengths away from any of the downward running tubes probably Ok. In both locations it would need to be a 1/4wave dipole and use the keel as the ground plane...of sorts. Question is how to mount so that the structural integrity of the keel isnt impacted. Not an issue for the rear as in most cases its has no real structural purpose once mounted on the base

 

Andy

 

 

[eastmeg2]

Hi Andy,

 

Structural integrity and certification were the reasons or choosing the rear keel extension to try mounting the aerial on. The was and is no way in I would ever drill a hole in the main keel section of a trike wing. The keel extension I'm using for the aerial mount for now is a shortened spare which I bought a full length replacement for last year.

 

In the past we've used a spec-an to retune CB aerials for airband, but that spec-an recently died. Also the SWR meter we use is a 27MHz unit, though still gives some indication. Any idea where an air-band SWR meter can be got?

 

Was just told yesterday that I can have 6m of RG58 from a friend's stash of known high-quality, with crimped male BNC connectors both ends. Unfortunately I'll still have to cut it and put some solder-on BNC connectors on - if I ever want to pack up my trike again which happens every time I fly the Buzzard since there's not enough hangar space for it fully rigged.

 

Cheers,

 

Glen

 

 

[Guest Andys@coffs]

Glen

 

An SWR Meter is a tuned item and is only of any use for frequencies in the bands that it covers. A CB SWR meter will be next to useless at VHF Airband frequencies unless it specifies that it will cover them.

 

Last I was looking for an SWR Meter I used Delsound in QLD as I worked there about 20+yrs ago and th owners then were both into Radio in a big way. They are also ICOM agents. That said a quick look at Ebay (US and Au) Showed heaps of options. Anything that doesnt specify a frequency range, dont touch with a barge pole. If there is a frequency range specified, get one that covers the whole band cause the usual way of tuning is to check at low end and then a high end, differences tell you whether you need to cut more off or add some to the antenna, obviously if it cant do the whole range you have less to play with....(P.S An ebay Item coming from Indonesia is one that I would personally stay well away from, both in context that I'd be amazed if it actually arrived and 2ndly unsure if its a fake/copy) If you did go that way be sure to use PayPal and or Credit card so that you can use the banks or Ebay to solve the problem if you get ripped off....Credit card leads by miles over PayPal if you can choose between the 2 PayPal only refunds if there is a credit balance in the sellers account I believe....if not too bad.

 

Regarding the size of the bands covered, I cant say for sure, but anything that specifies as a frequency band almost DC to almost Daylight is probably not going to be too great an SWR Meter as I said they are tuned and its very hard to tune a circuit to cover 2mhz to 500mhz. I suspect those that cover the Airband plus or minus 50-100mhz ie 50 to 150 will probably do a more accurate job and give you better results to work wih

 

In terms of using a Spectrum Analyzer to measure VSWR.....I guess Im not a practicing technician anymore so it doesnt immediately spring to mind how you could even do that with a spec-an... and in any event most people will baulk at the cost of an SWR meter... and turn blue and die when you metion the cost of a spectrum analyzer.

 

Re soldered vs Crimped. In my experience crimps are miles better, but a good crimper isnt cheap. for example the central pin crimper will use 3 or 4 pins to form the crimp, not just a general squeeze of everything. The shield crimp shoup be hex or octagonal and again not just a general squeeze. In fact in the RAAF crimpers were devices that were sent for calibration from time to time because if they arent done well, or the device is over agressive the resultant crimp will fail. In general if there is vibration at the connector then a crimp will be generally be superior to solder.

 

Andy

 

 

[eastmeg2]

Hi Andy,

 

The spec-an had a frequency sweeping signal generator in it, able to sweep from about 500kHz to 1.2GHz. The spec-an was being used as a scalar network analyzer to measure the absorption spectrum of the antenna, which we tuned for maximum absorption at approx 125MHz (Middle of the air band).

 

It was a rather old spec-an (late 70's I think), hence its recent passing away. 2nd hand it cost a few thousand in the late 90's.

 

Cheers,

 

Glen,

 

BE