jimbo wrote:
wrote:

On Mon, 20 Mar 2006 17:51:28 -0500, Dave wrote:


Bob Miller wrote:

SNIPPED

I'm confused, wouldn't it be more important to have the right
impedance and swr at the end of the feedline, where the transceiver is
expecting 50 ohms?

bob
k5qwg


SNIPPED

If you have 50 ohms at the antenna, then you will have 50 ohms
regardless of the length of coax.

A 3:1 VSWR, assuming other responses to this post are correct, means
that your antenna connection could be anything from 16 ohms to 150
ohms and all sorts of combinations of resistance and reactance within
that range.

An antenna analyzer will let you determine antenna resonance, antenna
feedpoint impedance [hopefully 50 ohms], coax line loss, and allow
proper adjustment at the connection points for your coax.

Once again, find someone in your area with an antenna analyzer. I
repeat my offer if you are within 60 +/- miles of Concord NH.




It's possible to tune a Jpole with just a SWR meter. I've done ti
many times though it's more tedious than with an antenna analyser.

The simplest step is add some length to the long section, not alot
and see how the change affects it. Add more or less as needed
for a good minimum. If the minimum is still not on the mark (1:1
is ideal but anything under 1.3:1 flies well) then adjust the tap
point (very small movements) and resweep again. With patience
you (the person that asked) will get it right.

What happens with Jpoles is they often are not built identical
and small differences do show. Also like most halfwave and
larger antennas being close to "stuff" tends to affect tuning.
No harm or foul but learning to prune (aka tune) an atenna is a skill
and worth developing.

Allison
KB1GMX


Adding some length to the long leg would be relatively easy, BUT moving
the feed point would be a real challenge! The coax shield and center
conductor are soldered to the ladder line wires. When I tuned a j-pole
in my work shop, tiny movements made a change amd maintaining the
position while trying to get it soldered was nearly impossible. (At
least for my clumsy fingers.) Is there a better way to connect the feed
point?

Thanks, jimbo
Connect your feed points to the Jpole using clamps. Adjust the location
of the feed point and the characteristic impedance by moving the clamps.
You do not have to adjust the length of the feed line until you fine
tune the feed point impedance.
I just finished making the collapsible jpole antenna that was featured
in QST last spring. When I first tested the impedance it was way off.
All I had to do was loosen the clamps and bump the clamps up and down
the pole to obtain optimal settings. Due to the out of ham band use I
will be putting the jpole to the SWR at my xmit freq. is higher than I
would like the midpoint SWR is about 1.1:1.
This is why Antenna design is an ART not a SCIENCE.

Dave WD9BDZ

OK, I misunderstood.

jimbo wrote:

Dave wrote:

jimbo wrote:

SNIPPED






Adding some length to the long leg would be relatively easy, BUT
moving the feed point would be a real challenge! The coax shield and
center conductor are soldered to the ladder line wires. When I tuned
a j-pole in my work shop, tiny movements made a change amd
maintaining the position while trying to get it soldered was nearly
impossible. (At least for my clumsy fingers.) Is there a better way
to connect the feed point?

Thanks, jimbo



Jimbo, you just added another variable to the discussion. Please
describe the details of the coax to ladder line and ladder line to
antenna for us.


Sorry for the confusion. The ladder line is the j-pole antenna. Cut to
the appropriate length, etc. The coax feed line is attached to the "J"
near the bottom, maybe 2 inches up.

Thanks, jimbo

jimbo wrote:
OK, I have a new 2 meter j-pole antenna installed in my 3rd floor attic.
I have 50 feet of LMR240 coax running to the basement. I measure SWR at
the following frequencies on simplex.

144.2 2.5
145.2 2.4
146.2 2.3
147.2 1.9
147.9 1.7

Can I conclude that the antenna is electrically short for the 2 meter band?

Thanks for any insight, jimbo


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

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?

You can tune a J-Pole antenna at one place in free space, move it
to another place in free space, and expect the same results. The
real world is not so forgiving. The conditions will not be
identical but they may (or may not) be close enough to be
acceptable to you. Welcome to the real world of ham radio.
--
73, Cecil http://www.qsl.net/w5dxp

On Tue, 21 Mar 2006 06:51:16 -0700, jimbo wrote:

In other words, is an antenna an antenna or is an
antenna and it's installation an antenna?

Hi jimbo

The second, the wire you call an antenna, its method of feed, and its
environment are the radiator. With care, it may work straight out of
the box, but your numbers already offer a different story; thus the
question becomes "what constitutes care?"

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?

The market for antennas destined to be put in an attic is very small,
hence you see very few for sale.

73's
Richard Clark, KB7QHC

"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.

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.

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

On Wed, 22 Mar 2006 06:22:12 -0700, jimbo wrote:

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

I have a ladderline j-pole right outside my attic, in a small tree,
and I lucked out on swr, just following the measurements I found on
the internet diagram of the thing. One trick you might try is to stick
a straight pin into the insulation, touching the longer wire element.
Raise and lower the pin for resonance.

bob
k5qwg

Must comment- that the answer is "MAYBE"! It depends on what is
UNDER that roof! If you have metal flashing (near vents, furnace
pipeing), or insulation with a "VAPOR BARRIER" usually, aluminium
foil backed, it may affect your ability to xmit a signal , and in
the case of the latter, might make a great "FARIDAY SHIELD" (signal
proof room)! And this would apply to comercial antennas as well!
Tho, the wood in a roof, and the roofing material should be
generally passive to even UHF, the other materials might be
a problem. Dont give up hope, as have friend witha antenna
(quad), on 6 meters, and 2 meters, in an attic dormer, if it had
been insulated correctly, would have been relatively RF Proof!
Might want to examine it before betting too complex! Jim -NN7K



"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

On Mon, 20 Mar 2006 08:12:51 -0700, jimbo wrote:

OK, I have a new 2 meter j-pole antenna installed in my 3rd floor
attic. I have 50 feet of LMR240 coax running to the basement. I
measure SWR at the following frequencies on simplex.

144.2 2.5
145.2 2.4
146.2 2.3
147.2 1.9
147.9 1.7

Can I conclude that the antenna is electrically short for the 2 meter
band?

Thanks for any insight, jimbo


That or it is mis-matched

--
73 for now
Buck
N4PGW