VHF Antennas on kingposts

[eastmeg2]

I've finally gotten around to installing the Microair in the instrument panel of the Buzzard and the Antenna on the keel extension.

 

It receives fine, but produces a modulated oscillation when transmitting. I suspect it's due to the keel extension not being very well connected electrically to the rest of the wing, producing a very poor ground plane. It seemed to improved when the motor was started but never completely went away - I suspect due to engine vibrations intermittently improving the electrical connection of the keel extension.

 

Len, I saw a keel extension mounted aerial on your trike but not sure if I remember if you did anything special to earth your keel extension tube, or am I imagining.

 

Cheers,

 

Glen

 

 

[cscotthendry]

Glen:

 

The type of antenna you're using determines whether you need to ground the keel extension or not. If you're using the antenna type that Airborne supply, it doesn't need a groundplane as it is the counterpoise type. If it's just a VHF wire whip, then it does need a groundplane. If you're using an antenna that requires a groundplane, then grounding the keel extension with a piece of wire will help some, but may not be enough. The groundplane for an antenna has to be a certain size and the keel extension may not provide enough metal to be an effective groundplane.

 

If you do have the antenna that Airborne supply (also available from Mobile One, who manufacture it) it doesn't need a groundplane. In that case, the noise you're getting may be related to the ignition on the engine.

 

 

[eastmeg2]

Hi Scott,

 

Thanks for your thoughts, there are photo's of the keel extension and aerial in post #8 of this thread.

 

I'll see how I go with grounding it and go from there.

 

 

[Tracktop]

Hi

 

May be a but premature but as this thread has surfaced again.

 

I have talked about my shadow problems before.

 

Flying in company with quite a few different trikes it appears that this effect ( heard by me) is

 

worst if communicating with new trikes. And completely absent on some that I have flow with.

 

SO not necessarily the receive on my trike but the transmit on others ( and mine).

 

Got me looking and thinking - that's dangerous

 

Last week crusty and I modified our aerials mounted on the king post.

 

Simply bending the king post top plate up to cause the aerial to sit away from the king post

 

instead of being parallel - shown in pic

 

I have noticed that on some original installations the aerial even angles in towards the king post slightly

 

We have had one flight together so far with no communication problems like we had experienced in the past.

 

Will keep you posted on our further evaluations and if this simple fix is really successful.

 

 

 

[alf jessup]

Well I had no problems with the transmit and reception with the wire antenna ontop of the kingpost on the ST 3 with it pointing upwards and the short flexible diapole down along the kingpost, this is what come with the trike when I got it and I had nothing but good reports about my radio from other aircraft in the 400 odd hrs i used it.

 

Now, SST wing on and original set up now on the right strut of the XT reception seems a little less good, mind you I have finally recieved in the mail today the anrenna set up that comes with the new sst wing as they had none when they sent it hence the temp set up from my old kingpost antenna, I still dont fancy the set up off the right under carrige strut but this is what they recommend, the difference is with the new gear from airborne it is 2 x fibreglass flexible rubber coated ariels the larger stiff and the short one flexible which will be good as the angle of the installation contacts the belly bag.

 

Hopefully the reception side of things will improve otherwise where too from here as there is no kingpost on the sst and I dont particicular fance mounting it on the pod at the front even though it would probably get less interference.

 

Will keep you posted on result after i fit it and get to fly it again once the weather calms down.

 

Cheers

 

Alf

 

 

[Mc Guyver]

I've finally gotten around to installing the Microair in the instrument panel of the Buzzard and the Antenna on the keel extension.It receives fine, but produces a modulated oscillation when transmitting. I suspect it's due to the keel extension not being very well connected electrically to the rest of the wing, producing a very poor ground plane. It seemed to improved when the motor was started but never completely went away - I suspect due to engine vibrations intermittently improving the electrical connection of the keel extension.

Len, I saw a keel extension mounted aerial on your trike but not sure if I remember if you did anything special to earth your keel extension tube, or am I imagining.

 

Cheers,

 

Glen

Hi Glen, the aerials that I use don't need any mods. I had a call from a Maitland flyer who was receiving me clearly in The Hunter Valley and with him using a hand held :-)

 

 

[Tracktop]

Hi Alf

 

Unfortunately hanger door height doesn't allow me to permanently install it aerial up. But I have on occasion turned it over for a trial.

 

I do fly with a couple of sst's ( aerial on strut) up here and I believe I have experienced a small amount of shadowing from them, but no where near as bad as the king post inverted aerial installation. No testing carried out (no scientific basis ), just my impression from numerous communications during flight in company.

 

Yes Len we can usually hear the Warnervale and Somersby traffic fairly clearly at Maitland if we tune in.

 

I haven't discounted going the rabbit ear direction yet.

 

 

[alf jessup]

Hi Ray,

 

Well time will tell on the strut as of this morning i removed the old one I had on the kingpost from the strut where I had it installed while waiting for the new on to arrive,

 

Only have a short lenght of cable now instead of 4 meters coiled up with this new one.

 

All I am waiting on now is the weather to calm down as it is blowing it's butt off here and the SST is itching to go flying, hmm maybe that is me itching to go.

 

I will let you know the results as soon as I can get airborne.

 

Cheers

 

Alf

 

 

[eastmeg2]

I forgot to mention that the volume of the modulated oscillation remained constant when I adjusted the radio volume during transmission - so it's potentially an issue with the microphone pre-amp I'm using, which I'm going to lower the gain on at the next opportunity.

 

 

[Mc Guyver]

Hi AlfUnfortunately hanger door height doesn't allow me to permanently install it aerial up. But I have on occasion turned it over for a trial.

I do fly with a couple of sst's ( aerial on strut) up here and I believe I have experienced a small amount of shadowing from them, but no where near as bad as the king post inverted aerial installation. No testing carried out (no scientific basis ), just my impression from numerous communications during flight in company.

 

Yes Len we can usually hear the Warnervale and Somersby traffic fairly clearly at Maitland if we tune in.

 

I haven't discounted going the rabbit ear direction yet.

Hi Ray, If you did the aerial would be below the top of your King Post and no problem getting it in or out of the hangar :-)

 

 

[cscotthendry]

Hi AlfUnfortunately hanger door height doesn't allow me to permanently install it aerial up. But I have on occasion turned it over for a trial.

I do fly with a couple of sst's ( aerial on strut) up here and I believe I have experienced a small amount of shadowing from them, but no where near as bad as the king post inverted aerial installation. No testing carried out (no scientific basis ), just my impression from numerous communications during flight in company.

 

Yes Len we can usually hear the Warnervale and Somersby traffic fairly clearly at Maitland if we tune in.

 

I haven't discounted going the rabbit ear direction yet.

Ray:

 

The counterpoise antenna base has a "live" side and a "dead" side. The dead side is the side that the rubber-duckie whip is usually mounted to. If you just swap the rubber duckie and the fibreglass whip without turning the base over, you probably won't get much benefit from it. The Mobile One people say that the antenna is not intended to operate that way so you might even make things worse. Also, as a counterpoise antenna mounted on the kingpost, the kingpost alters the balance between the counterpoise and the live side of the antenna. It would definitely give better performance with the live side pointing up instead of down, but that causes problems for people with low hangar doors and it's still not the best setup. I thought your angled mount was a pretty good solution. You might need to remove the fibreglass end when you pack the wing up though.

 

I have moved my antenna to the SST position on the right strut. It is far from ideal, but better than the kingpost mount. I still have a shadow problem as another pilot reported he didn't hear my base call when he was behind me downwind, but overall, I think it's a better setup.

 

 

[Tracktop]

When I tried it inverted ( the correct way up ) I did invert the mount as well :thumb_up:, but good to highlight that that needs to happen.

 

In my testing efforts I managed to loosen ( break ) the mounting base. This probably happened when I bent the mount bracket without removing the aerial base from the bracket first. The aerial still works but I have a new base ready to install next visit.

 

Removal of the longer aerial section will be required to pack the wing up but that is just a matter of easily unscrewing it from the base.

 

I also wonder if just a slight angle out ( perhaps 70mm) may have been sufficient rather than the larger angle I chose, but then I did want to try and fix it so went for more rather than less.

 

 

[eastmeg2]

Here's a couple of shots of the Buzzard radio installation I took on the weekend. I don't think I'll be fitting much more onto the panel. There's a matchbox-sized microphone pre-amp velcroed and cable tied to the side of the radio behind the panel. and I suspect it's the source of the oscillation.

 

I've seen i couple of Airborne SST's with similar antenna mountings on their keel tube extensions.

 

Panel still looks a bit untidy as I'll be modifying the gain in the mike pre-amp as soon as I can get some more bits from Jaycar and make another visit. Then I put the trim around the edges again.

 

Cheers,

 

Glen

 

 

 

 

[Mc Guyver]

Here's a couple of shots of the Buzzard radio installation I took on the weekend. I don't think I'll be fitting much more onto the panel. There's a matchbox-sized microphone pre-amp velcroed and cable tied to the side of the radio behind the panel. and I suspect it's the source of the oscillation.I've seen i couple of Airborne SST's with similar antenna mountings on their keel tube extensions.

 

Panel still looks a bit untidy as I'll be modifying the gain in the mike pre-amp as soon as I can get some more bits from Jaycar and make another visit. Then I put the trim around the edges again.

 

Cheers,

 

Glen

Hi Glen,

I have machined up an aluminium bracket for the keel which takes the Vee Rabbit brilliantly.

 

It works for the topless and all others as well.

 

I am now getting brilliant reception:clap: and transmission:clap:

 

 

[alf jessup]

Im not happy at all with the reception and transmitting of the new aerial on the undercarridge strutt as when I transmit it effects my skydat by shooting the egt's off the scale and making the alarm flash on while the button is pressed, i think the power generated by the radio coming out of the antenna is radiating to the instrument module box on the top of the engine.

 

I might turn over the antenna so the long one is on the bottom but i will have to use my old wire one as I can bend it to shape so it does not hit the ground as the new fiberglass one will be too long.

 

McGuyver do you have any details on this mounting out of the keel tube extension, do you cut down an extension tube so it does not stick out as far, also would you clamp it or drill out the extension tube, I am also gathering this would only be suitable for the V rabbit aerial.

 

My new aerial that i got with the sst wing works nothing like my old one ontop of the kingpost on the ST3 wing, frankly i think it was a waste of money.

 

Cheers

 

Alf

 

 

[Mc Guyver]

Im not happy at all with the reception and transmitting of the new aerial on the undercarridge strutt as when I transmit it effects my skydat by shooting the egt's off the scale and making the alarm flash on while the button is pressed, i think the power generated by the radio coming out of the antenna is radiating to the instrument module box on the top of the engine.I might turn over the antenna so the long one is on the bottom but i will have to use my old wire one as I can bend it to shape so it does not hit the ground as the new fiberglass one will be too long.

 

McGuyver do you have any details on this mounting out of the keel tube extension, do you cut down an extension tube so it does not stick out as far, also would you clamp it or drill out the extension tube, I am also gathering this would only be suitable for the V rabbit aerial.

 

My new aerial that i got with the sst wing works nothing like my old one ontop of the kingpost on the ST3 wing, frankly i think it was a waste of money.

 

Cheers

 

Alf

Hi Alf,

This bracket is for the Vee Rabbit.

 

It is cast aluminium and fits into the wing keel.

 

 

[Guest davidh10]

My two penneth worth..

 

I'm new to the site and have been looking at some of the interesting threads when I happened across this one. I'm new to flying, so not much experience to give there, but I've been involved with electronics and radio for many years (although not recently), so thought I could add to the discussion..

 

As an amateur radio operator, I designed and built many of my own antennas. Both HF and VHF. Note that I do not claim to be a professional antenna designer. Rather than say what I agree and disagree with, I'll just state some points:-

 

• VSWR of 1 is ideal, however 1.5 is not bad for low power transmitters, and that is what we're talking about here. It is difficult to get the ideal, if for no other reason that you won't know you have hit the best value until it starts going up again, and its really hard to cut it longer ;)
   
   
  
• VSWR Meters aren't tuned for a specific frequency, however they do have a designed frequency range over which they will be useful. If it does not have a spec that covers the frequency range that you want to measure, it isn't likely to be much use and could even be contributing to the VSWR in its own right.
   
   
  
• Using a tuning chart, supplied with an antenna assumes the antenna is installed in the ideal manner with appropriate groundplane and no interfering metalwork around it. If this is not the case, then using a VSWR meter to adjust by trimming the length is the only way of being certain.
   
   
  
• We're talking here about an unbalanced feed (co-ax), where the signal is on the inner core and the shield / braid is grounded. If the antenna is not tuned correctly (ie VSWR = 1) then there will be radiation from the braid of the co-ax as well as the antenna. In this situation, if the co-ax is electrically grounded at other than the transmitter or run along metal structures, it will alter the tuning of the load seen by the transmitter and the radiation pattern. Hence, reflected power damaging the transmitter final is not the only reason that an ideal VSWR is desirable. In this case also, having radiation from the co-ax shield means it is part of the tuned circuit, so if you change its length, the VSWR will also change. Just another reason for ensuring that the antenna is tuned.
   
   
  
• The antenna being talked about that has a quarter wave whip and a rubber ducky emanating from opposite sides of the antenna base does not require a "plate" ground plane. The tuned "rubber ducky" looks electrically like a metal disk of quarter wavelength radius at the operating frequency. The rubber ducky part is actually screwed to the chassis as well as the co-ax braid and the quarter wavelength whip above is the "driven element". This could be mounted with the base level with the top of the King Post or alternatively at the rear of the Keel, with the rubber ducky pointing groundwards. A little birdy tells me that Airborne has very recently copied a setup like this based on seeing it implemented on one of their agent's trikes (with SST wing).
   
   
  
• The "Rabbit Ears" antenna claims a gain of 2.5dB. There's no free ride, so for this to be true it must have a non-uniform radiation pattern. In other words it has some directionality, which is undesirable in this application.
   
   
  
• Having a driven element parallel to the King Post and less than several multiples of a quarter wavelength away from it will affect both the radiation pattern (directionality or shading) and the tuning (VSWR). If its location is changed, the antenna needs to be re-tuned.
   
   
  
• A quarter wavelength whip should have a groundplane as close to the base as possible. The groundplane should have infinite size, but in practical terms a quarter wavelength radius disk is good and four quarter wavelength horizontal radial arms suffices quite nicely. IMHO, the antenna with the tuned rubber ducky opposing the quarter wavelength driven element is a more elegant and lower drag solution.
   
   
  
• A properly tuned antenna will provide better reception as well as better transmission.
   
   
  
• According to antenna theory, a coat hangar won't work as an antenna. So why do we often see this innovative design on some older vehicles? Ideals aren't always absolutely necessary, and in any case most public broadcast stations transmit 50KiloWatts.
   
   
  
• Co-ax cable comes not just in different impedances but in differing loss per length grades. A solid dielectric causes greater loss at VHF than Hf, so often low loss co-ax has a foam or spoked structure, so there is more air than plastic. This reduces loss, but makes the co-ax more susceptible to physical distortion by bends or tight cable ties. Distortion will cause a change in impedance, which in itself will cause an increase in VSWR.
   
   
  

 

 

So after all that, what do I use? Well I've only recently purchased my trike, second hand, and have not yet had a really close look at the antenna. The antenna looks like a centre fed halfwave dipole mounted with the centre, level with the top of the King Post and the co-ax run almost against the bottom half, which is stood off the King Post about 2cm. If this is accurate, then it is far from ideal. The radio is an Xcom and there's at least a spare metre of co-ax coiled under the dash. The latter is also not ideal. I haven't measured the VSWR.

 

I do get reports that my signal is good and crystal clear. A few days ago at 5,000' I heard a "Warnambool Traffic' call from Yarrawonga (approx 400km), so you'd have to say that the reception side is working well, even with some obviously good skip conditions. This being the case, I haven't had cause to examine it closely.

 

Hope this is helpful:big_grin:

 

 

[Tracktop]

I might turn over the antenna so the long one is on the bottom but i will have to use my old wire one as I can bend it to shape so it does not hit the ground as the new fiberglass one will be too long.

My new aerial that i got with the sst wing works nothing like my old one ontop of the kingpost on the ST3 wing, frankly i think it was a waste of money.

 

Cheers

 

Alf

Hi Alf

 

You should be able to invert your aerial without it hitting the ground.

 

Waited until I went to the hanger to get my facts straight.

 

Took a few pic this morning to show how they come out of the factory on the SST's in our hanger.

 

Note the coke can has clearance under the aerial tip.

 

From memory there is a join in the cable near the steering foot peg pivot and a short piece of cable to the radio - only just enough long enough though - no extra.

 

 

 

 

 

[alf jessup]

Ray,

 

You have tundra tyres and I dont, anyway i fixed the problem and the radio works 5 times better now that with the airborne strut set up.

 

I got a block of nylon about 4 inches long machined up exactly the the same as the keel extension post where it goes into the keel.

 

It is held in place by the pin that holds the keel extension when inserted into the keel, I also tapered it down about 10mm towards the rear which bought the small aerial away from the prop and ran with the original cabling that use to run up to the kingpost.

 

The antenna is now away from from the electronic interference aroung the engine and now there is also no shadowing.

 

It cost me 60 bucks to get a machine shop to make it and it is the best 60 bucks I have spent.

 

I will try and get a couple of photos of it tomorrow and send them to you, trust me it is a far better set up than on the strut (atleast it is for me)

 

Reception and transmit is crystal clear now.

 

Cheers

 

Alf

 

 

[Tracktop]

Hi Alf

 

Pleased to see you got some good results

 

Some pics of your install would be good for the SST guys up here to consider

 

 

[cscotthendry]

Hi AlfPleased to see you got some good results

 

Some pics of your install would be good for the SST guys up here to consider

And for the rest of us too. I installed my antenna as per the SST setup except mine has the active end pointing upwards to prevent ground strike (I fly from a grass field). Since then I've been plagued with ignition noise in the receiver. I think I will move my antenna to a keel extension too.

 

 

[alf jessup]

Ray & Scott,

 

I just been flying and didnt take my camera, I am just heading to watch my son play footy then off to Melbourne to take my mum down for her herart surgery tomorrow,

 

Will be back later tonight so will try and get some photos of it tomorrowe( monday) before i head offshore for my 2 weeks.

 

Cheers

 

Alf

 

 

[eastmeg2]

We got the problem with te modulated oscillation sorted out today.

 

Firstly, the things which did not fix it.

 

1. Earthing the keel extension tube.

 

2. Lowering the gain of the microphone pre-amp by about 4dB.

 

What did work was creating some space between the radio and the microphone pre-amp (Aluminium) box. As I had velcroed and cable tied the pre-amp box onto the left side of the radio there must have been some capacitive feedback. There is now a 1/4 inch thick slab of wood with velcro either side, in between.

 

Subsequent airborne radio test gave 5/5.

 

From a couple of km away in the air I had a clear conversation with another trike on the ground next to the hangar.

 

The panel is also now looking much tidier with the trim around the outside again gracing its edges.

 

JR, the old M760 is living the good life again . . .

 

Cheers,

 

Glen

 

 

[alf jessup]

Ray / Scott,

 

Just wondering if you have made up and tried out the keel block for the aerial and what your results have been if you have, me I am very happy now with mine.

 

Would be interested to hear from you.

 

Cheers

 

Alf

 

 

[cscotthendry]

Alf:

 

I managed to find a supplier of nylon rod and I just need to get the dimensions so that I can machine it. I will probably have to wait until I go flying next, and given the weather at the moment...thumb_down

 

I've also been thinking of another option, a swivel mount on the front of the pod. I'd want it to fold back so I can get the covers on, but then swing out in front. The design of the thing is still just rough so I won't bother with the details yet.