On Nov 28, 10:31*am, Tom Horne wrote:
On Nov 27, 2:53*am, Owen Duffy wrote:









Tom Horne wrote in news:8b22e1d5-907a-4ee6-ab40-
:

...

Replies are welcome on or off list. *Let me say that I would rather
not be subjected to any tirades about the inferiority of the J-pole
approach or the superiority of some other antenna design. *That does
not mean that I am not open to suggestions for a better approach as
long as it is civilly stated and I am spared the application of Rant
Mode.

It is a sad statement of the state of affairs when you feel a need to
compose a help request with such conditions, 40% of your post is setting
conditions on your would be helpers responses.

This is not unusualy, the posts can often can be paraphrased as "I have
got this really good idea, and I don't want to hear from anyone that it
is less than a really good idea... over".

Your 40% conditions is by no means a record, higher percentages have
been observed on QRZ and eHam, such high percentage as to make the whole
thing look like a troll rather than a genuine request for help.

My own view is that it helps to be a little humble when seeking help.

Owen

Owen

I apologize for coming across as arrogant. *I didn't mean to.

Perhaps it would help to know that I was looking at the J-pole as a
transportable antenna that would be used when the operating position
is down in between buildings, ridges, mountains, etc. *I did look at
the online patterns for a J-pole. *When I used a diamond ground plane
under those conditions the results were less than satisfactory. *The
intent is to get the signals up out of the hole. *The aluminum conduit
was because it is much lighter than copper pipe and far less expensive
at present pricing. *I may want to replicate the antenna several times
if it happens to overcome the problem for the purpose of reliable
communications outside of simplex two meter range without limiting the
operator pool to general and above licensees. *Also there are several
places were we could use six meter APRS to collect data from, for just
example, a temporary stream gauge by installing the gauge in the time
between a flash flood watch and any subsequent flash flood warning.
Obviously the temporary gauging point may well contribute to the
decision as to whether and when to issue the warning. *I was thinking
to use rigid aluminum conduit because of the availability of both
compression and threaded couplings. *I intended to test which coupling
technique would stand the most bending force on the upper portion of
the antenna. *I already guessed that the antenna might need to be
guyed but I wanted to try whether using a one inch or even one and a
quarter inch nominal size would avoid the need for that. *Since the
Arrow Antenna design is fed at the bottom of the shorter matching stub
I thought it might be easier to transport and deploy without damaging
it. *As for keeping the stub in line with the radiator I had
envisioned a short piece of 5/8" fiberglass U channel with two
stainless steel pipe clamps as being robust enough to withstand
repeated handling. *The three quarter wavelength portion of the
antenna would be fashioned into three nearly equal sized sections for
ease of transport.

I have constructed a dual half wave collinear J-pole using copper
pipe. *It had given me much better empirical performance than the
borrowed ground plane that I had used before it. *If I could get the
needed range out of the simple J-pole I was next going to work on the
physical challenge of building another collinear dual half wave for
six meters were the second have wave could be added readily for use
when the antenna would not be located below local horizon.
--
Tom Horne, W3TDH

The love-hate relationship with J-poles has been pretty well
documented over the years here on r.r.a.a. My feeling is that you can
resolve that pretty well by decoupling the antenna from the feedline,
and also from other nearby conductors. That's something you want to
do with any antenna, at least where you want to have consistent,
reliable performance in the face of differing feedline and antenna-
support installations. One of the implications here is that you'll
have a non-conductive support for the antenna. You should be able to
get pretty reliable feedline decoupling with a choke in the feedline
where it drops below the antenna, and another a quarter wave below
that. For narrow-band operation (say operation in a band less than
1MHz wide at 50MHz), I'm partial to tuned chokes: a small coil of
coax bridged by a capacitor to resonate it at the operating
frequency. That's obviously not the only way to do it, but done
right, it can give you quite a high impedance in series with the line
while adding negligible additional loss due to the slightly longer
feedline.

Since a J-pole is nominally 3/4 wave long, at 6 meters wavelength that
implies something roughly 4.5 meters, or about 14 feet, long. I guess
you can come up with some arrangement of pipes that will fit across
the back seat of your car that you can put together to get what you
want, maybe four nominally 1/4 wave sections. I wonder, though, if it
wouldn't be easier to use telescoping sections of fiberglass (or
similar reasonably stiff mast material) that would take you up to
where the top of the antenna needs to be, and then just use a wire
antenna. Then if there happens to be a natural way to hang the
antenna (e.g. from a line tossed over a tree limb), you don't need to
bother with setting up any mast + support.

As far as exact dimensions of the antenna itself, given all the good
antenna analysis programs out there, I consider that the easy part,
but dependent on just how you elect to make it: wire or sections of
conductive tubing, and just how you will feed the antenna (tapped up
the shorted quarter wave bottom section, or connected to the open
bottom of the quarter wave). Pick a mechanical arrangement that works
for you, then use a program to come close to the required feed
arrangement and details of the element lengths and spacing, then build/
measure/trim to get what you want -- if you accept that anything below
a 1.5:1 SWR works well, then you will probably have little if any
trimming to do, assuming you modeled it accurately.

Cheers,
Tom