VHF Com. Ant. for Biplane

Hello
A young friend of mine is in the process of restoring an EAA Biplane ,
last weekend we started working on his new engine mount , he started to
discuss the fact that he is installing his VHF Com. antenna inside the
fuselage , this experimental is the older tube and fabric design , cockpit
is about 24 in. wide and narrows to the tailpost , being an old HAM radio
operator , a lot of red flags went up , he want's to mount a standard 1/4
wave vertical somewhere aft of the cockpit area , width in the proposed spot
would be about 14 in. , all of my experience with radiating elements says
that it will not work , I think the element will try to couple with the tube
structure , SWR will go sky high and it will be a no go , I will bring an
MFJ 259 to the airport next weekend and try to give him a graphic demo. of
my perceived doubts .
I know that the guys with the glass and wood are doing OK with there
internal Ants. , wondering if someone else has had any luck with getting RF
to exit a 4130 airframe , he wants it to use in the standard 118- 136
portion on the band , plane has an alum. turtledeck , I told him that the
deck should make a good gnd. plane for the 1/4 wave vert. , he is concerned
about aesthetics , any thoughts would be appreciated .
Thanks
Phil
W8KLK

"Phil" wrote in message
...
Hello
A young friend of mine is in the process of restoring an EAA Biplane ,
last weekend we started working on his new engine mount , he started to
discuss the fact that he is installing his VHF Com. antenna inside the
fuselage , this experimental is the older tube and fabric design , cockpit
is about 24 in. wide and narrows to the tailpost , being an old HAM radio
operator , a lot of red flags went up , he want's to mount a standard 1/4
wave vertical somewhere aft of the cockpit area , width in the proposed
spot would be about 14 in. , all of my experience with radiating elements
says that it will not work , I think the element will try to couple with
the tube structure , SWR will go sky high and it will be a no go , I will
bring an MFJ 259 to the airport next weekend and try to give him a graphic
demo. of my perceived doubts.

Your 259 may or may not show the actual problem. The actual problem is that
the steel tube fuselage is an excellent Faraday shield and almost all of the
radiation will wind up warming the steel tubing. We've tried it and tried
it every way from Sunday and it just will NOT work.






I know that the guys with the glass and wood are doing OK with there
internal Ants. , wondering if someone else has had any luck with getting
RF to exit a 4130 airframe , he wants it to use in the standard 118- 136
portion on the band , plane has an alum. turtledeck , I told him that the
deck should make a good gnd. plane for the 1/4 wave vert. , he is
concerned about aesthetics , any thoughts would be appreciated .

The steel tubing will make a better groundplane unless the turtledeck is
electrically fastened to the tubing. Again I say unto you, getting RF out
of a steel tube fuselage is an exercise in futility.

Jim

RST Engineering wrote:
"Phil" wrote in message
...
Hello
A young friend of mine is in the process of restoring an EAA Biplane ,
last weekend we started working on his new engine mount , he started to
discuss the fact that he is installing his VHF Com. antenna inside the
fuselage , this experimental is the older tube and fabric design , cockpit
is about 24 in. wide and narrows to the tailpost , being an old HAM radio
operator , a lot of red flags went up , he want's to mount a standard 1/4
wave vertical somewhere aft of the cockpit area , width in the proposed
spot would be about 14 in. , all of my experience with radiating elements
says that it will not work , I think the element will try to couple with
the tube structure , SWR will go sky high and it will be a no go , I will
bring an MFJ 259 to the airport next weekend and try to give him a graphic
demo. of my perceived doubts.

Your 259 may or may not show the actual problem. The actual problem is that
the steel tube fuselage is an excellent Faraday shield and almost all of the
radiation will wind up warming the steel tubing. We've tried it and tried
it every way from Sunday and it just will NOT work.






I know that the guys with the glass and wood are doing OK with there
internal Ants. , wondering if someone else has had any luck with getting
RF to exit a 4130 airframe , he wants it to use in the standard 118- 136
portion on the band , plane has an alum. turtledeck , I told him that the
deck should make a good gnd. plane for the 1/4 wave vert. , he is
concerned about aesthetics , any thoughts would be appreciated .

The steel tubing will make a better groundplane unless the turtledeck is
electrically fastened to the tubing. Again I say unto you, getting RF out
of a steel tube fuselage is an exercise in futility.

Jim

I use a rubber ducky aviation handheld antenna on the belly of our
RV-4 with good results. Maybe that would be less harmful to his
sensitivities than a quarter wave whip ??

Bob Olds
RV-4 , N1191X
Charleston,Arkansas