Just wondering what people's opinion is on this... I'm trying to
decrease the overall mounted size of a jpole design. In the past I've
always tried to keep the bottom 1/4 wave stub section well above the
mast top. Just for the heck of it, I started testing with the stub
below the top of the mast (mast blocking the bottom 1/4 wave stub).
The antenna is still electrically isolated from the mast of course...
but what I found is that I was able to mount the antenna to the mast
and have it go up the first 1/4 wave stub part of the antenna at least
2/3 of the way, before there was any SWR problem at all. In fact,
there was virtually no difference in the SWR doing that, as compared
with having the very bottom of the antenna well above the mast top.
Can anybody think of any other adverse effect in doing this? On a
fairly low band it reduces the overall size of the antenna
significantly, so it's definitely an advantage to mount it lower,
assuming there's no adverse effect on the performance. The SWR doesn't
seem to change at all until you start getting the top of the mast up
near the actual 1/2 wave portion of the antenna.

Thanks for any help,

wrote in message
...
Just wondering what people's opinion is on this... I'm trying to
decrease the overall mounted size of a jpole design. In the past I've
always tried to keep the bottom 1/4 wave stub section well above the
mast top. Just for the heck of it, I started testing with the stub
below the top of the mast (mast blocking the bottom 1/4 wave stub).
The antenna is still electrically isolated from the mast of course...
but what I found is that I was able to mount the antenna to the mast
and have it go up the first 1/4 wave stub part of the antenna at least
2/3 of the way, before there was any SWR problem at all. In fact,
there was virtually no difference in the SWR doing that, as compared
with having the very bottom of the antenna well above the mast top.
Can anybody think of any other adverse effect in doing this? On a
fairly low band it reduces the overall size of the antenna
significantly, so it's definitely an advantage to mount it lower,
assuming there's no adverse effect on the performance. The SWR doesn't
seem to change at all until you start getting the top of the mast up
near the actual 1/2 wave portion of the antenna.

Thanks for any help,

There may be a problem with mounting hardware detuning the stub.

wrote:
Thanks for any help,

The rule is that a conductive support needs to be kept
a few diameters away from the spacing of the non-radiating
stub. If that condition is met, it should be OK.
--
73, Cecil http://www.w5dxp.com

In article
,
wrote:

I'm trying to
decrease the overall mounted size of a jpole design.

Xray-

Why not use the top of the mast as part of the antenna?

You could either measure 3/4 wavelengths down from the top and bolt-on
the 1/4 wave stub, or measure 1/4 wave down and bolt-on a 3/4 wave main
element!

You could run the coax inside the bolt-on element, and out through a
hole at the matching point.

Fred
K4DII

wrote in message
...
Just wondering what people's opinion is on this... I'm trying to
decrease the overall mounted size of a jpole design. In the past I've
always tried to keep the bottom 1/4 wave stub section well above the
mast top. Just for the heck of it, I started testing with the stub
below the top of the mast (mast blocking the bottom 1/4 wave stub).
The antenna is still electrically isolated from the mast of course...
but what I found is that I was able to mount the antenna to the mast
and have it go up the first 1/4 wave stub part of the antenna at least
2/3 of the way, before there was any SWR problem at all. In fact,
there was virtually no difference in the SWR doing that, as compared
with having the very bottom of the antenna well above the mast top.
Can anybody think of any other adverse effect in doing this? On a
fairly low band it reduces the overall size of the antenna
significantly, so it's definitely an advantage to mount it lower,
assuming there's no adverse effect on the performance. The SWR doesn't
seem to change at all until you start getting the top of the mast up
near the actual 1/2 wave portion of the antenna.

That's interesting. My experience is related to yours, although what I did
was bring the mast close to the entire long part of the "J" and found I had
to get within an inch before I saw any measurable effects, rcv or xmit. I
did it with a 2m copper J.

In your situation, possibly the quarter-wave section is a tank circuit and
would need proximity of a mast section of size close to its resonant freq to
be detuned by it. I don't know ... just musing. Others will mug me for
getting it wrong and that's OK.

More musing: Does the entire "J" have to be upright? Can't I put an elbow
where the quarter-wave and half-wave sections meet, so the radiating element
is vertical, but the quarter-wave section is horizontal? (In the case of a
tower side-mount, this would position the radiating section a nice distance
from the tower. Inverted OK.)

"Sal"

Sal M. Onella wrote:
More musing: Does the entire "J" have to be upright? Can't I put an elbow
where the quarter-wave and half-wave sections meet, so the radiating element
is vertical, but the quarter-wave section is horizontal? (In the case of a
tower side-mount, this would position the radiating section a nice distance
from the tower. Inverted OK.)

Sal, I think that the position of the J affects its polarization. Around
1995, I did some informal experiments with an MFJ 300 ohm twinlead J-Pole
and 2m SSB. I used a Yaesu FT-290RII with 2 watts output.

At 2m, polarization is important and I found that by laying the J-Pole
horizontaly, I was able to extend the range of my signals, even if
I layed the antenna on a wood picnic table.

I did not experiment with the antenna in a mixed orientation as you describe
above.

Another posibility is to place a 300 ohm J-Pole antenna inside a rigid
PVC pipe. If the pipe is long enough, the base can be clamped without
affecting the antenna. This does move the feed line from being extended
horizontaly at the feed point to being extended verticaly downward, but
this did not seem to effect the antenna, and the MFJ unit I have is
designed that way.


The 450 ohm ladder line units I have made are built with a horizontal
feed, but that's more lazyness than anything else.

73,

Geoff.




--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM

More musing: Does the entire "J" have to be upright? Can't I put an elbow
where the quarter-wave and half-wave sections meet, so the radiating element
is vertical, but the quarter-wave section is horizontal?

No, it doesn't, and yes, you can.

The quarter-wave matching section may radiate somewhat (it's not a
perfectly-balanced transmission line) but the effect should be
minimal.

You'll still want to mount the antenna so that the quarter-wave
section remains some distance from the mast.

You can extend this approach a bit further, and make a fully- balanced
version of a bent J-pole - a.k.a. a "double zepp". This would be two
half-wave radiating sections, fed through a quarter-wave matching
section.

The coax is connected along the matching section, using a 4:1 coaxial
balun.

A further variant of this is an "extended double Zepp" (or "double
extended Zepp"), which is two 5/8-wave radiators with a somewhat
shorter matching section.

For both the standard and extended double Zepp antennas, the use of a
balanced feed allows the "inner" end of the matching section to be
grounded to the mast - it can even be part of a metal mast, or can
have extensions addd to allow it to be clamped to the mast.

See http://www.tcarc.ca/public/2mdez.php for complete plans for the
extended double Zepp. The simple "double Zepp" can be made in a
similar fashion, with shorter radiators and a longer matching section.

I built a 440 version of the EDZ for use as a linking-radio antenna
for a local repeater system. It works nicely. One thing I'd do a bit
differently - I'd actually do the matching (selection of feedline
attachment point and shorting bar) after mounting it on the tower,
rather than in free space - the tower mast does have enough of an
effect on the impedance to change the SWR a bit.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Thanks for the info... I'm should have added that the one I was
working with was contained in RF transparent plastic tube. So that's
how I can mount the whole thing right next to the mast. Maybe if you
were using a traditional copper model, it might be different. But with
the one I have, there was no difference with the mast next to the stub
section. Though I have noticed similar commercial models do seem to
leave an air gap between the mount area and the element.

Cecil Moore wrote:
wrote:
Thanks for any help,

The rule is that a conductive support needs to be kept
a few diameters away from the spacing of the non-radiating
stub. If that condition is met, it should be OK.

essentially, that stub is a openwire balanced transmission line, so all
the usual rules about spacing of openwire line from things that will
perturb the field apply. As Cecil says.. a few spacings away..