VHF Antennas on kingposts

[cscotthendry]

I made some modifications to Alf's design and came up with this. I haven't measured the trike yet, but I'm hoping this design will remove the need to have the antenna angled to clear the prop. The angle bracket is bolted to a piece of 10mm metal bar buried in the nylon block. The heads of the bolts will be safety wired a-la the prop securing bolts.

 

 

 

 

[alf jessup]

Scott,

 

Should work fine mate, one thing for sure is it will work 10 times better than on the strut and that i will have a small wager on lol.

 

Cheers

 

Alf

 

 

[cscotthendry]

Alf:

 

I don't know what I did with the pictures you sent. With the one you made, how close does the rubber-duckie get to the prop?

 

Here's hoping that it is better than on the strut. As I mentioned before, I get ignition noise there. Also I get transmit shadowing on the port side.

 

 

[alf jessup]

Scott,

 

On mine the bottom rubber duckie can get to 1mm from the prop tip at its lowest point (bar fully forward) but that is only if you bend the rubber duckie back towards the prop tip hence that is why i angled it slightly forward.

 

If you go straight down like in your one it will be close to the prop arc and about an inch or more below the tip with a great possibility of being sucked into the prop.

 

With mine as soon as the bar comes off the front strut the duckie is well above the tip of the prop.

 

The only reason i put a bit of sash cord from the lower part of the flexible duckie to the keel wires at the rear was, 1. a bit of redundancy in the off chance the prop could suck the flexible part of the duckie back into the 1mm tip of the prop when the duckie is at its lowest point (control bar fully forward (not at all likely to happen) and 2, it will stop the antenna from undoing and would also prevent it going through the prop if it did some how happen to fall off.

 

I put a little thought into it before i made it.

 

Cheers

 

Alf

 

I can resend you the photos when i get back home if you wish to relook at them

 

 

[cscotthendry]

Alf:

 

I am hoping that by making the distance between the end of the keel tube and the point where the antenna mounts shorter, the rubber-duckie will clear the prop without having to angle it. As you can see from the pic, I can make it shorter still, but if necessary, I still have the option to angle it like yours. I've got enough nylon rod to experiment with it a bit.

 

I'll probably also use your idea of the restraining cord as a safety.

 

 

[alf jessup]

Scott

 

It's all trial and error and i am sure you will work it out fine and i am also sure you and the boys up there will be far happier with the performance of the radios with this set up than with them set up on the strutt.

 

Cheers

 

Alf

 

 

[cscotthendry]

I ended up changing my design yet again. Here's the 3D model and the finished article. I won't know until this weekend whether it clears the prop OK or not. The mount plate is bolted to a threaded metal rod embedded in the nylon block. The bolts pass through the rod and into the nylon. The holes in the nylon have been drilled smaller than the bolts so that they act like the retainers in nyloc nuts.

 

Send me an email if you'd like a copy of the drawings.

 

 

 

 

[eastmeg2]

Hi Scott,

 

That's the "Ducks Guts", (ie, GOOD). Assuming you won't have any prop strike issues and being a di-pole has no ground-plane requirement.

 

I've managed to borrow an air-band power & SWR meter which I used on the weekend to find I'm getting a SWR of 1.2 (Very good) and power output of 4 watts which is typical of the earlier Microair M760's. But I did find that the wing top wires were shadowing transmission and reception in front of the trike - this shouldn't be a problem with the SST though. Air-air conversations with another trike 10km in front were readable but scratchy, and much better and clearer when I turned 90 degrees (side on) to the other trike 10km away.

 

Cheers,

 

Glen

 

 

[alf jessup]

Scott,

 

Looks good mate, short and stubby so it shouldnt be an issue i dont think with a prop strike, hope you happy with your reception.

 

BTW i am using a microair 760 with my set up and it has proven to work fine, i still havent put on the fibreglass aerial that came with the strutt mount kit, I had it on the strutt but it didnt work as good as the original wire one i had when it was on the strutt..

 

Cheers

 

Alf

 

 

[cscotthendry]

Scott,Looks good mate, short and stubby so it shouldnt be an issue i dont think with a prop strike,

Alf:

 

That's how it's supposed to work, in theory. I'll know this weekend if it works in practice. I also have the M760 and have been reasonably happy with it except for the noisy reception I get with the antenna on the strut. Fingers crossed this move will solve that little problem. I see from Glen's post that he still has shadowing issues in the front with his antenna mounted at the keel. Unfortunately, this whole 'moving the antenna' business was intended to fix the shadowing problems. We'll see what turns up.

 

BTW, as an old avionics tech, we always used to say the RF side of things is FM*

 

(*FM = Magic)

 

 

[alf jessup]

Scott,

 

I dont seem to be suffering anymore shadowing since i moved the antenna from the strut to the keel and I am betting you will have the same result.

 

Cheers

 

Alf

 

 

[cscotthendry]

Alf:

 

Here's hoping! I thought that the extra distance between the top wires and the antenna might reduce or get rid of the shadowing, but given what Glen said, I started to wonder.

 

It certainly has to be better than having the active part of the antenna lying alongside the kingpost the way it came from the factory.

 

The downside for me is that I have to remove my wing for hangaring and that means removing the antenna to put the keel extension on. A small enough price to pay for the safety of good radio comms, I think.

 

 

[Mc Guyver]

Hi Scott,

 

I use the Vee Rabbitt aerial which attaches to your kingpost. No need to remove it excepting when de rigging completely.It is fitted below the top of the kingpost so does not get damaged in and out of low hangars.The aerial performance is amazing and has no shadowing problems due to its design. I have fitted mine on a special bracket attached to the wing keel (topless wing)

 

 

[alf jessup]

Scott,

 

I guess i wasnt thinking when i said about the shadowing effect with the kingpost being there as I am topless now and dont have the kingpost or wires to worry about, but mind you when I had my old set up on the ST3 wing i had no shadowing effect on mine as the long aerial was pointing up off the top of the kingpost factory mounting bracket and the rubber diapole one was pointing down.

 

McGuyver

 

My next step if this set up i used didnt work satifactory was going to be the v rabbit aerial like you have as I have heard they are quite good.

 

Cheers

 

Alf

 

 

[cscotthendry]

Mc Guyver:

 

I looked back through this thread to see if you had posted pics of your setup. It doesn't appear that you have, yet I feel Like I've seen it or something similar. If you haven't posted any pics, how about it? I'd love to see what it looks like, and I'll bet others would too.

 

 

[Tracktop]

I am not dissatisfied with my aerial setup inverted on the king post since I angled it out. Seems to be working fine with no more shadowing evident and good transmit and receive performance.

 

 

[cscotthendry]

Ray:

 

A friend of mine who is fluent in the magic of RF described the action of an antenna thus: "Imagine a ball travelling along your antenna from root to tip, and as it travels, it gets bigger until it reaches the tip and then it pops off the antenna." In the context of you not having the shadowing since you angled your mount, this kind of makes sense to me. With the antenna lying alongside the kingpost, as the "ball" gets bigger, more and more energy would be absorbed by the kingpost. With the antenna angled, it would allow the "ball" to expand as it goes. This may be a clumsy analogy, but it makes sense (of an insensible topic) to me.

 

 

[Mc Guyver]

Mc Guyver:I looked back through this thread to see if you had posted pics of your setup. It doesn't appear that you have, yet I feel Like I've seen it or something similar. If you haven't posted any pics, how about it? I'd love to see what it looks like, and I'll bet others would too.

Hi Scott,

 

Attached are 2 pictures. As can be seen I have a strobe mounted on the bracket as well.

 

 

 

 

[cscotthendry]

McGuyver:

 

That looks really good. Do you get any interference with the strobe so close to the antenna?

 

BTW I tried my new mount this weekend and

 

(a) With the bar pushed fully forward, the rubber duckie could contact the end of the prop. So I'll be re-making the mount with the angle and

 

b) I mis-measured and the mount was a 10mm longer than it should have been and protruded an extra 10mm out of the keel. I don't think the extra 10m (when removed) will be sufficient clearance without the angle, so I'll be putting that 10 degree angle in.

 

 

 

[cscotthendry]

Here's the MKII version with (hopefully) the correct dimensions this time. It also has the 10 degree angle to move the rubber duckie end further from the prop.

 

 

 

[Guest GordonM]

G'day Scott,

 

not sure if you solved you problem with the antenna but I am sitting here having a look at some of the microlight posts (windy outside today)

 

We mount our antenna on the drag link or forward suspension arm pointing down. This way you have a short co-ax run and no need to remove the connection when removing the wing something I am sure we have forgotten to do on at least one occasion (DOH!!) the reception is not exceptional whilst on the ground but I guess you have to ask the question do you really need to speak to someone 200 miles away when taxiing? (or even in the air for that matter)

 

[ATTACH]12100.vB[/ATTACH]The reception in the air is the same as a correctly mounted antenna on a kingpost.

 

Do NOT under any circumstances use the trike as a ground, trikes are horrible at producing ‘noise’ ether through poor wiring technique and through loose cables (loose side wires whilst taxiing will cause all sorts of static over your transmissions.) Airborne still insist on fitting the voltage regulator/rectifier about as far away from the motor as possible and this run of cable (and extra earth point) causes transmission issues as well. Have a look at the set up in the pic and you can see that it will easily cable tie to the strut. You can make your own quite easily but make sure that the counterpoise is insulated from the trike but sheathing it in plastic.

 

 

 

[cscotthendry]

Gordon:

 

That's a very interesting looking antenna, having the counterpoise at right angles to the active end. I've never seen one like that before.

 

I did mount my antenna on the drag link, but with the active end upwards. It was mounted on a bracket made by Airborne for the trikes that have the strutted wings. Since I regularly fly from grass strips, I was concerned about a ground strike on the stiff end causing it to break if it was mounted upside down. Also,having the antenna that close to the engine caused a fair amount of ignition noise when the received signal wasn't very strong. Obviously that was caused by the AGC trying to boost the signal and also boosting the local noise. I didn't have the antenna insulated from the trike ground as you suggested, but the antenna manufacturers also suggested that it's a good idea.

 

Since I mounted the antenna on the keel, I have very good, noise free reception, even with weak signals. I spoke to Shane Duncan at Airborne and he showed me their version of the keel mount. They made their's from tubing and I think they will start selling them at some stage. The downside of the keel mount is that you have to remember to connect the antenna and also you have to take it out of the keel when raising the wing on the mast or the counterpoise will hit the mast and get bent (D'Oh!).

 

I hear you about having the reg so far from the power source in the engine! I guess there's always trade-offs in any design though.

 

Where did you get that antenna from?

 

 

[Guest GordonM]

Hi Scott,

 

I have to admit that we make them here and sell them via Sky Sports Innovations. It took quite a bit of testing to figure out a simple solution to a common problem. We even made a simple 'butt plug' style aerial that fitted rather neatly into the base bar but it was a tad too labour intensive to produce.

 

A lot of pilots seem to want to be able to blast the airways with a zillion watts of power when in reality all you really need is clear transmission and reception--simple.

 

Fitting a 'pi filter' onto the power feed to your comms and radio will also assist in reducing reception and transmission noise problems created by poor wiring and the standard issues with regulator/rectifiers.

 

It is not often that a pilot gets to hear what their transmissions sound like and you would be surprised at how many bad transmissions never get back to the pilot. We set up a ground recording facility so that we could compare and analyse several different 'in flight' transmissions and sort the good from the bad.

 

I also operate from a grass (if we ever get rain) paddock but the short aerial does not create a problem.

 

 

[Kev]

Ray,

 

I have tried the 'vee rabbit' antenna on the kingpost. It works very well and does not require a ground plane.

 

The only issue is that it needs to be removed for derigging the wing. It also increases drag a little.

 

Safe Flying

 

Kev

 

 

[Mc Guyver]

Kev what you need to do is make a short keel extension for the wing and mount the aerial horizontaly, then all that is required is to pull out a pin and disconect the aerial. You should be able to do that in under 1 minute and yes it does work just as well in the horizontal:clap: