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Mounting windscreen-antenna on center cabin brace

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Mike Whisky

Joined: 05 Jun 2006
Posts: 336
Location: Switzerland

Posted: Tue Feb 24, 2009 1:21 pm Post subject: Mounting windscreen-antenna on center cabin brace

Hi,

I am planning to tape /glue Vans windscreen COM antenna to the front of the center cabin brace.

For those who used it as well and are flying, where did you put yours?



I want to paint or put a black cover / tape over it.



Please let me know what you think.



Cheers

Michael

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#511

recapen(at)earthlink.net
Guest

Posted: Tue Feb 24, 2009 1:42 pm Post subject: Mounting windscreen-antenna on center cabin brace

I put mine in that place on my 6A and made it longer to make sure it would work.  I planned to have it checked by a bunch of HAM operators (also pilots) that wanted to come see what I had built.



They hooked up their meters to it and told me that it was electrically too short for the freqs that I wanted to use.  They also indicated that the center cabin brace could interfere with the radiation pattern of the antenna.



My recommendations:



Make it extra long - then trim it later after you get the SWR checked (with everything else installed and running).



Invite the radio guys to come out and do the checking (unless you're a radio guy too).



I'm still trying to figure out how to re-do mine...I do have more copper tape - it's just the PITB factor....meanwhile, glad I have two radios for phase 1 flying



Ralph

RV6A N822AR (at) N06 4.7 hrs



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nukeflyboy

Joined: 05 Jan 2008
Posts: 162
Location: Granbury, TX

Posted: Sat Feb 28, 2009 4:55 am Post subject: Re: Mounting windscreen-antenna on center cabin brace

Michael,

I think that is not a good idea for a couple reasons.  That steel post will interfere with the radiation pattern making transmissions in some directions problematic.  It is also your receiving antennae and will have the same challenge.  To work at all it will have to be mounted on the plexi in front of the post and will look like hell.  Do not underestimate the radiofrequency interference that it can generate which causes problems with many electronic devices (engine monitors, EFDs, etc.) which will be directly below it.  It works acceptably on a bubble canopy if you are in a RV-6 and looking for a low cost alternative to a commercial comm antenna.  It is not a good choice for an RV-10.



do not archive

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Dave Moore
RV-6 built and sold
RV-10 built and flying

AirMike

Joined: 27 Feb 2007
Posts: 514
Location: Nevada

Posted: Sun Mar 01, 2009 12:29 am Post subject: Mounting windscreen-antenna on center cabin brace

I was also told that propagation can be poor on the center post. I was advised by Aerotronics that a conventional antenna is the best bet. I picked up a cheap used Cessna antenna at OSH for $30. Experimenting with the NAV antenna from Vans in the area just inside the windscreen. It runs across the front under the upholstery. I ran the coax up thru the center post. I was also told that VOR reception might be poor, but want to give it a try anyway. Some have had good luck with the Bob Archer stuff

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Q/B - sold.

carl.froehlich(at)cox.net
Guest

Posted: Sun Mar 01, 2009 6:42 am Post subject: Mounting windscreen-antenna on center cabin brace

I plan on installing a simple dipole VOR antenna in the inside groove of the

cabin top, feed via coax running up the inside of the center post.  The

antenna elements will be #18 wire (or whatever fits the groove the best).

The approximate length of the antenna follows the standard formula:  Length

(ft) = 468/frequency (in MHz).  So for VOR reception, the frequency range is

108.1 through 117.95 MHz.  Using 110 MHz, antenna length (both sides total)

= 4.25 ft, or 51 inches, or 25.5 inches on each side of the center feed

point at the center post.



Some considerations:

1.  Dipoles have the max current, min voltage point at the center.  This

means the interaction of the center post is minimal.  I do not expect any

noticeable performance degrade over something like a VOR antenna mounted on

top of the vertical stabilizer.

2.  As the antenna elements will follow the contour of the cabin top, the

antenna is more like and inverted 'V', thus the typical nulls you would have

off the ends of a dipole are reduced (in this case 090 and 270 relative to

the fuselage axis).

3.  I'll start with 27" or so of wire on each side of the center feed point,

trimming the ends after installation using an antenna tester to find the

best resonance point over the full frequency range.

Carl Froehlich

RV-8A (450 hrs)

RV-10 (still on fuselage)

KV4U

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Bob Turner

Joined: 03 Jan 2009
Posts: 885
Location: Castro Valley, CA

Posted: Sun Mar 01, 2009 3:28 pm Post subject: Re: Mounting windscreen-antenna on center cabin brace

"2. As the antenna elements will follow the contour of the cabin top, the 

antenna is more like and inverted 'V', thus the typical nulls you would have 

off the ends of a dipole are reduced (in this case 090 and 270 relative to 

the fuselage axis). "



I think your logic is off a little here. The 'end on' sensitivity of such an antenna is only to vertical polarization. VOR and Localizer signals are horizontally polarized. Expect trouble when the station is off to the side.

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