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).

First, send me the model of the instrument that you used for the
measurement. Then explain why the trap in the antenna didn't totally mess
up the VSWR for the VHF band. You DO understand that an ELT antenna is a
trap monopole, don't you? You DO understand that the top 2/3 of the antenna
is decoupled from the bottom end by an LC trap, don't you? That a properly
operating ELT antenna should be 2:1 or less at 121.5 MHz. and more than 10:1
above 123 and below 119 MHz?

My bet is that you used a CB power meter, good buddy, 10-4.

Of course, you could be measuring a 51 ohm resistor that somebody put in the
antenna to "match" the antenna across the band. That would let the antenna
radiate about as well as a limp piece of spaghetti in a copper septic tank.

Jim




"Scott" wrote in message
.. .
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

See my other post about the equipment used to measure power.

What LC trap? This antenna is a straight, flexible whip. There isn't
ANY coil in the whip. 51 Ohm resistor? Could be, who knows...but the
antenna radiates and receives fine.

The antenna routinely talks air to air over 70 miles or more out here in
the midwest where the hills aren't inflated like the hills and egos out
in Grass Valley appear to be.

Thanks for blowing my whole image of you. You just lost all of the
respect I used to have for you.

Scott Littfin
Bloomer, WI

RST Engineering wrote:

First, send me the model of the instrument that you used for the
measurement. Then explain why the trap in the antenna didn't totally mess
up the VSWR for the VHF band. You DO understand that an ELT antenna is a
trap monopole, don't you? You DO understand that the top 2/3 of the antenna
is decoupled from the bottom end by an LC trap, don't you? That a properly
operating ELT antenna should be 2:1 or less at 121.5 MHz. and more than 10:1
above 123 and below 119 MHz?

My bet is that you used a CB power meter, good buddy, 10-4.

Of course, you could be measuring a 51 ohm resistor that somebody put in the
antenna to "match" the antenna across the band. That would let the antenna
radiate about as well as a limp piece of spaghetti in a copper septic tank.

Jim




"Scott" wrote in message
.. .

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

Here's a link to the ELT antenna picture (the exact antenna I am using
across the com band). You can get as abusive and abrasive and you want.
Tell me theory all day long and it still doesn't matter. The fact is,
it WORKS in my installation and has been for about 7 years now.

http://www.chiefaircraft.com/Aircraf..._05-02-006.gif

Tell me more about the LC trap on this antenna.

Scott Littfin



RST Engineering wrote:

First, send me the model of the instrument that you used for the
measurement. Then explain why the trap in the antenna didn't totally mess
up the VSWR for the VHF band. You DO understand that an ELT antenna is a
trap monopole, don't you? You DO understand that the top 2/3 of the antenna
is decoupled from the bottom end by an LC trap, don't you? That a properly
operating ELT antenna should be 2:1 or less at 121.5 MHz. and more than 10:1
above 123 and below 119 MHz?

My bet is that you used a CB power meter, good buddy, 10-4.

Of course, you could be measuring a 51 ohm resistor that somebody put in the
antenna to "match" the antenna across the band. That would let the antenna
radiate about as well as a limp piece of spaghetti in a copper septic tank.

Jim




"Scott" wrote in message
.. .

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