Antenna placement question.

mvadney

gyroboy
Thanks Paul.

I have seen the antenna tuner that you mentioned. They usually go for $300. And since it does sound like I will need to trim the antenna I think that I will get one. Pricey, but if that eliminates the possible noise issues then I have no problem with that. That makes total sense the if the signal cannot radiate properly from the antenna that the coax start leaking to near by components. Now I just have to understand the ground plane concept and add it in a way that doesn't look bad and doesn't cause me to repaint my pod.
 

Gyro28866

David McCutchen
Find the seller on Ebay, which I mentioned. Send him a message. He will build the antenna to your specs. He was easy to work with. RG8 will increase the price a little, but it is worth the better shielding.
I had him trim mine to 22.8 inches for a 1/4 wave on the radiating element/antenna and specified the length of coax with a BNC connector.
I mounted the antenna vertically down under the rear of my pod. I added a piece of metal sheeting in the lower section of the pod to create a counter poise/ground plane. The combination of this created an SWR of 1.1:1 while on freq. 122.8 MHz .
However, if you are using a handheld radio, the rubber ducky antenna should be sufficient for several miles.
 

PW_Plack

Active Member
Mike, yes, that's the current version of the analyzer I've had for about 25 years. Be sure to get the MFJ-7737 UHF-to-BNC adapter, which will allow you to plug the coax lead from the radio right into the analyzer. If you order from GigaParts, the adapter is here:

https://www.gigaparts.com/nsearch/?q=BNC+to+PL259

The BNC female on the top of the analyzer is an output for a frequency counter, not the input!
 

mvadney

gyroboy
Thanks Paul.
I appreciate the help

Can you describe a small ground plane that I could use if mounting an antenna under the keel? Does the direction of where the antenna is pointing make much difference?
 

PW_Plack

Active Member
Mike, the last ground plane I built for the aircraft band was made from CB parts I got at a truck stop, with aluminum foil tape for a ground plane. The vertical element was a plain stainless steel whip with the little ball on the end, starting about 23" long before trimming. It was held at its base by a set-screw adapter to a 3/8"-by-24 threaded antenna base. The base itself was a feed-through type used on those 90º angled mirror brackets, but it was used without the angle bracket, and just run through a fender washer for support behind a fiberglass panel. The mount had a CB-type female coax connector on the side inside the panel, but you might be able to find one with a BNC connector, the aircraft standard.

I believe the length when we got done testing and trimming was about 22-1/4 inches for resonance centered at 120 MHz, and we got the SWR to about 1.4. Your length will vary based on your unique frame and mounting location.

To be sure it responds as a vertically-polarized antenna, get at least the bottom third of it coming straight down from the bottom of the pod before any bend. Keep the tip at least 10" from metal frame members if you can. If it gets closer, the resonant frequency will be lower.

It doesn't make much difference which way the bent portion points from a performance standpoint, but it may be a little quieter in the wind if it's bent back rather than forward.

These really simple antennas may be prone to collecting high static voltages as they fly. Check with your radio manufacturer to see if that will be a problem. Some radios can continuously discharge this static to ground. Others may be damaged by it.

I recently moved, and all my radio stuff is in stacked plastic tubs. I'll post some photos of an antenna made this way when I can find it.
 
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mvadney

gyroboy
Thanks Paul. I also was communicating with the MFJ company that makes the antenna analyzer too. They were recommending a MFJ-1728B 5/8 wave model. I was also surprised to see that the ball at the end of an antenna actually served a purpose. I always thought that it was just making the end safer in the real world.
Wonders never cease.
 

mvadney

gyroboy
On Aircraft Spruce's site I found a fairly short antenna. LYNX MICRO SYSTEM SHORT FLEXIBLE VHF ANTENNA
What do you think?
 

500e

Member
Would think this is a tung in cheek response

"Will this antenna work inside a carbon fiber aircraft like the Lancair Legacy?
Yes, this antenna will work through wood & fabric and composite aircraft like carbon fiber."

To me CF would be at least a partial shield \ reflector.
We are using the flexi style antenna masts on ground vehicles with good results, if mast screws on the base fitting some form of locking could be required,
We use this for Fiberglass roofed vehicles, there must be a source in the US

http://www.co-star.co.uk/prod/51/alu...dth-300mm.html

http://www.co-star.co.uk/prod/52/alu...idth-75mm.html

Another type we use bottom of page the ground plane base, not a brilliantly engineered device a fiddle to keep the rods in place when fitting in a cramped space

http://www.wallen-antennae.co.uk/deltamagnetic.html
 
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PW_Plack

Active Member
mvadney;n1131767 said:
They were recommending a MFJ-1728B 5/8 wave model.
That's an uneducated attempt to upsell you on their part!

A 5/8-wave vertical antenna achieves signal "gain" measured at the horizon by redirecting energy that would normally be sent straight up or down. That's fine for land mobile, where energy radiated straight up is normally wasted. In aircraft, you need a good signal up and down, too!

The 5/8-wave is also more than double the length of the 1/4-wave, even longer than a dipole, despite the loading coil at the base, and still needs a ground plane for proper operation.
 

PW_Plack

Active Member
mvadney;n1131770 said:
On Aircraft Spruce's site I found a fairly short antenna. LYNX MICRO SYSTEM SHORT FLEXIBLE VHF ANTENNA
What do you think?
It hurts my head to pay that much, but there's no reason it couldn't work well. It will have lower efficiency than a 1/4-wave whip due to losses in the coil, but shouldn't be bad. I've never seen, used or measured that antenna.
 

Terry_Smith

Gyroplane Fan
No Title

Well, I saw this post and since I am going to be in a similar situation, I thought I'd get some suggestions. My Sportcopter pod will have an MGL EMS, so like Mike, I'm looking for an alternate location for an antenna. Also, since I'm going to have a GPS antenna on the pod, it is recommended that it isn't near an VHF radio antenna. One option that I'm considering is a ground independent antenna from Microair. I was planning on mounting it on the main gear strut - I've attached a picture to show an approximate location.

It will actually sit about 1-2 inches behind the strut, with no other frame parts nearby. I've seen a picture of a Sportcopter in Australia with something similar, so I know it's been done before. Just curious of the pros/cons of this. I'll be writing Sportcopter for their thoughts as well. I didn't want it too close to the mast, as there is a lot more metal at that point. Thanks for any comments. Have a great Easter Weekend!
 

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