RadioBanter - View Single Post

March 22nd 06, 01:22 PM posted to rec.radio.amateur.antenna

wrote:
On Tue, 21 Mar 2006 22:15:22 -0800, "Sal M. Onella"
wrote:

"jimbo" wrote in message
...

jimbo wrote:

Thanks for all of the advice. I have one more question. Can I tune a
J-Pole antenna in my shop and then move it to the attic and expect the
same performance? In other words, is an antenna an antenna or is an
antenna and it's installation an antenna?

It would seem to me that if the antenna is tuned and works in my shop
it should work in the attic. Otherwise, how do "store bought" antennas
work?

Thanks, jimbo

I have built about twenty copper-pipe j-poles and I have found that with
five feet or so between the antenna and surroundings, the tuning doesn't
change enough to be annoying.

The copper pipe j-pole can be dropped into a pipe and hoisted into the clear
for VSWR evaluations if you really want to get gnats-assey about it.

I've found getting them 6ft (for 2m ) above the ground was enough that
another 20ft didn't change it enough to measure.

Speaking of that, I have found that j-poles usually don't hit 1:1, even
when the low point is mid-band. However, after I get the best match,
I insert a 100 pF capacitor between the coax center pin and the antenna
and "Viola!" I stole that idea from the gamma match.

I've had good swr match(1:1 or unmeasurably close) without the cap.

Also the original poster has the ladder line based jpole in the attic.
If it's like most I've seen the attic is within 5ft . Like I'd said
elsewhere the twinlead and ladder line versions are fussier and
tend to be different from one batch of wire to another.

Allison

Yes, the attic has the roof within five feet. No walls but lots of
cross braces. I am going to tune a J-Pole in my shop and then take it
to the attic and check the difference.

jimbo