Aircraft antennas

Believe it or not (I checked with trusty wattmeter), I get good SWR
across the entire Comm band using a cheap ELT antenna I bought from
Chief Aircraft for about $35 several years ago (the one that has a very
flexible whip). I have one mounted on the top of the (leading edge)
fairing that covers the wing joint on my Corben Junior Ace The fairing
basically is the leading edge, (filling the gap between wing panels) and
goes ffrom front spar top to front spar bottom. The metal is about 6"
wide, so it would seem a bit small for a ground plane, but it works. I
can routinely talk air to air over about 70 miles with both planes at
pattern altitude. Air to ground (from 1000 feet) is 20-30 miles or so.

Scott


wrote:

Orval Fairbairn wrote:

In article ,
"ccwillwerth" wrote:


Hi, I am about ready to cover my Cub type airframe, but need a place to
attach a com antenna. I was considering brazing a plate to the airframe so
that it would be just under the fabric. The antenna is the type that has a
ceramic insulator on the bottom of the antenna that insulates the stainless
steel antenna from the airframe.


Charlie, you will get several opinions, but here is mine.

If at all possible, get a "broadband" VHF Com antenna, not a wire whip.
The "broadband" fiberglass antennas have a VSWR of less than 2:1 across
the range of 118 to 136MHz, while the metallic whip will have an
bandwidth of only about 5Mhz where the VSWR is below 2:1. At the band
edges, the VSWR will be high enough to cause the VSWR protection
circuitry in transistorized transmitter to shut the output power down
to nearly zero. Although the wire-whip can be cut&tuned to just Unicom
frequencies (122.7 to 123.6 Mhz), it could be marginal for transmission
at some ATC frequencies. The whip will work ok for receiving even at
the band-edges, because the receiver doesn't care about the VSWR.

If your fuselage is anything like my Piper PA20, there are enough metal
tubes to act as a ground plane without adding any additional conductive
material, other than a mounting plate. I would put the antenna base on
a metal plate which is just below the plane formed by the fabric.
Radius the edges of the plate so that the fabric doesn't ride on a
sharp edge. The plate could be long enough to bridge between two
fuselage cross-brace tubes, but only about 4 to 6" wide. It must be
electrically "bonded" to the cross-braces, so to avoid drilling holes
in the cross-braces, your idea of welding some attach "ears" to the
cross-braces to mount the plate is good.

Not.

Jim



"Scott" wrote in message
.. .

Believe it or not (I checked with trusty wattmeter), I get good SWR across
the entire Comm band using a cheap ELT antenna I bought from Chief
Aircraft for about $35 several years ago (the one that has a very flexible
whip).

I can send you forward and reflected power readings if you wish, say
every 1 MHz from 118 to 136. Now, with that said, that is measured on
the ground, not in flight where the whip curves back from all the high
speed flying at 75 MPH

Scott


RST Engineering wrote:

Not.

Jim



"Scott" wrote in message
.. .


Believe it or not (I checked with trusty wattmeter), I get good SWR across
the entire Comm band using a cheap ELT antenna I bought from Chief
Aircraft for about $35 several years ago (the one that has a very flexible
whip).

Charlie: I recently installed a VHF com antenna on the tailboom frame of my
Baby Belle helo. It's dimensions are 16" X 12" Which are less than the
length of the antenna. I ran an antenna check using an MFJ 250 antenna
analyzer and the highest VSWR I got was 2.2. This was so much better than
the bent antenna that had been on the belly of the ship which did have a
large ground plane but had VSWR greater than 3 and in some cases as high as
5. One thing you can do is try out a prototype ground plane and then have a
Ham or someone with the right equipment do a check.

--
Stuart Fields
Experimental Helo magazine
P. O. Box 1585
Inyokern, CA 93527
(760) 377-4478 ph
(760) 408-9747 publication cell
"ccwillwerth" wrote in message
...
Hi, I am about ready to cover my Cub type airframe, but need a place to
attach a com antenna. I was considering brazing a plate to the airframe
so that it would be just under the fabric. The antenna is the type that
has a ceramic insulator on the bottom of the antenna that insulates the
stainless steel antenna from the airframe. Does the antenna need a large
plate for a ground plane or will a small plate be sufficient? If a ground
plane is required, can the copper foil tape be used on the inside of the
fabric as the ground plane?

Charlie

Oops. The installed ground plane dimensions are 16"X12".
--
Kathy Fields
Experimental Helo magazine
P. O. Box 1585
Inyokern, CA 93527
(760) 377-4478
(760) 408-9747 general and layout cell
(760) 608-1299 technical and advertising cell

www.vkss.com
www.experimentalhelo.com


"Stuart & Kathryn Fields" wrote in message
.. .
Charlie: I recently installed a VHF com antenna on the tailboom frame of
my
Baby Belle helo. It's dimensions are 16" X 12" Which are less than the
length of the antenna. I ran an antenna check using an MFJ 250 antenna
analyzer and the highest VSWR I got was 2.2. This was so much better than
the bent antenna that had been on the belly of the ship which did have a
large ground plane but had VSWR greater than 3 and in some cases as high
as
5. One thing you can do is try out a prototype ground plane and then have
a
Ham or someone with the right equipment do a check.

--
Stuart Fields
Experimental Helo magazine
P. O. Box 1585
Inyokern, CA 93527
(760) 377-4478 ph
(760) 408-9747 publication cell
"ccwillwerth" wrote in message
...
Hi, I am about ready to cover my Cub type airframe, but need a place to
attach a com antenna. I was considering brazing a plate to the airframe
so that it would be just under the fabric. The antenna is the type that
has a ceramic insulator on the bottom of the antenna that insulates the
stainless steel antenna from the airframe. Does the antenna need a
large
plate for a ground plane or will a small plate be sufficient? If a
ground
plane is required, can the copper foil tape be used on the inside of the
fabric as the ground plane?

Charlie

ccwillwerth wrote:
Hi, I am about ready to cover my Cub type airframe, but need a place to
attach a com antenna. I was considering brazing a plate to the airframe so
that it would be just under the fabric. The antenna is the type that has a
ceramic insulator on the bottom of the antenna that insulates the stainless
steel antenna from the airframe. Does the antenna need a large plate for a
ground plane or will a small plate be sufficient? If a ground plane is
required, can the copper foil tape be used on the inside of the fabric as
the ground plane?

Charlie



Our experience is that anything inside a structure especially metal is
NOT going to work. As one other said... put the aerial on the outside
where it's meant to go.

Also we have just added a table on VSWR effects and power drop off with
a bad aerial, check http://www.mcp.com.au/xcom760/faq/faq.html for
details... right down the bottom of the page (All the way down)

Interesting if you have a bad aerial Regards Michael from XCOM

"XCOM" wrote

Our experience is that anything inside a structure especially metal is NOT
going to work. As one other said... put the aerial on the outside where it's
meant to go.


With careful reading, I take that he is asking about putting JUST the ground
plate under the fabric, with the aerial on the outside, as you suggest.
--
Jim in NC