Antennas in PVC pipe
Jack Taylor
12 solid wire and planned on enclosing it inside a length of 1 in dia
PVC pipe. Using a grid dip meter, cut the antenna to resonate on 224.
However upon inserting it inside the pipe, the resonant frequency dipped
about 8 MHz lower! Went ahead and trimmed it some more until resonance
inside the pipe was reached at 224 MHz. In this case at least, looks
like PVC affects the VF of the physical dimensions by 0.91 as compared
to free space.
Has anyone else on the group have experience with placing an antenna
inside of PVC pipe?
73 de Jack
John Passaneau
Hi Jack:
I've never used PVC pipe over an antenna, but I know the insolated wire
does make a big differance. I thought I'd be smart and I substuted a
lenght of #12 PVC insolated house wire for the stainless steel whip on a
2 meter mobile antenna, as it would be easer to trim to frequecy, and I
would then just transfer the lenght to the steel whip. I did that and
recheck the SWR and it was 4 to 1. It turned out that it was about 3 in
too short and I had to buy a new steel whip and do it over. It was a good
(?) example of dielectric loading. This happens at all frequencies, but
is more noticable at higher frequencies.
John Passaneau, WB8EIY, State College, Pa.
Rane Curl
Rane Curl <rane...@engin.umich.edu> wrote:
>(vacuum is 1.0), so the capacitance is increased, and the resonant
>frequency is lowered - by a *factor* of 4.3, in fact, if we lived in a
By a *factor* of SQRT(4.3) - sorry about that. Should think first...
Rane N8REG
Rane Curl
Wolfgang Gellerich <gell...@kafka.informatik.uni-stuttgart.de> wrote:
>In article <3573@NJ7P>, n7...@azgate.nj7p.ampr.ORG (Jack Taylor) writes:
[...snip...]
>|> However upon inserting it inside the pipe, the resonant frequency dipped
>|> about 8 MHz lower! Went ahead and trimmed it some more until resonance
>|> inside the pipe was reached at 224 MHz. In this case at least, looks
>|> like PVC affects the VF of the physical dimensions by 0.91 as compared
>|> to free space.
>As far as I know, the PVC molecules can become "activated" by certain
>frequencies, and PVC is generally not considered to be a good choice
>as material to construct antennas from.
Well, not "activated", but the dielectric constant of PVC is ca. 4.3
(vacuum is 1.0), so the capacitance is increased, and the resonant
frequency is lowered - by a *factor* of 4.3, in fact, if we lived in a
solid PVC universe (hi). PFTE ("teflon") would cause a smaller effect
as its dielectric constant is ca. 2.1.
Rane Curl N8REG
Jim Fellows
My question is, what about other "plumbing" materials for supports.
Such as a boom for a 435 helix or yagi made of ABS. Elements not inside support.
I'm not sure about the correctness of the microwave test method, and know of no
other way of testing.
Jim