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Author Topic: tape measure antenna SWR>25  (Read 4804 times)
KC0UXC
Member

Posts: 8




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« on: December 29, 2005, 06:37:00 PM »

I just built the tape measure antenna, but when I tested it with th MFJ SWR analyzer, I get a SWR>25.  I double checked the antenna and everything looks good.  Is there a minimum length on the feed line? Any ideas would be appreciated....
Chuck
KC0UXC
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WB6BYU
Member

Posts: 13578




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« Reply #1 on: January 03, 2006, 12:33:46 PM »

An SWR that high indicates a feedline problem - either a
short or open somewhere.  (Or trying to use a 2m antenna
on a different band.)

A quick test would be to measure the continuity across
the coax connector - if you are using a matching stub it
should look like a short circuit to a DC meter.  Removing
the matching stub it should be an open circuit for DC.
(This requires that the driven element be split in the
middle.)

Perhaps the most likely problem if all else looks OK is
getting a good connection to the metal blade of the tape
measure.  Sticking a wire under a hose clamp won't do it.
My prefered method is to scrape off the paint and solder
the coax leads directly to the inside corners of the
driven element.  An alternative is to use a sheet metal
screw through the element and into the PVC fitting
(which needs to be pre-drilled.)  I also scrape the
paint away from the hole and use lock washers with
Ox-Gard on the screw to maintain a good contact.  (Oh,
and the hole needs to be punched, not drilled, or the
tape measure material will be weakened.)  

My current assembly method is fiberglass-reinforced
strapping tape around the ends of the PVC couplers to
hold the elements on, plus one or two screws in each
element if they are doubled in the middle for added
rigidity.  Since the driven element is not continuous,
I always use one screw in each side - this is convenient
for attaching the coax, but soldering is better for
long-term service.


Other than that, nothing in the general antenna should
give you such a high SWR.  Generally I start with the
elements cut to resonant length, add the matching stub,
then trim the ends of the driven element for best match.
But even this shouldn't go over 3 : 1 at worst.

And, no, there is nothing magic about the length of the
cable that would cause such a high SWR.
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AD5TD
Member

Posts: 114




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« Reply #2 on: April 05, 2007, 11:45:19 AM »

This antenna uses a "hairpin" match.  It will appear as it is a dead short.  You may have to unhook the coax ans check it first.

I built this antenna and it works FB.  Had to do a bit of tuning but not much.
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WA3QNS
Member

Posts: 2




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« Reply #3 on: March 21, 2009, 11:13:15 PM »

Same problem here, using electrical tape to secure
elements to PVC, and an added an 7 inches of PVC on each
element for rigidity. Coax not open or shorted, SWR
with or without shorting stub is off the chart!
This antenna was built solid and with care, guess there is a first time for everything. Maybe its that cheap Tandy coax!
ciao
wa3qns

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WB6BYU
Member

Posts: 13578




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« Reply #4 on: March 22, 2009, 08:23:54 PM »

WA3QNS -

How did you attach the feedline to the driven element?
I've used screws through the element (with the mylar and/or
paint scraped off underneath, plus lock washers and
OxGard on the joints.  Still had problems with poor
connections over time.  In fact, on a recent hunt one of
the transmitters always measured 90 degrees off in the
same direction.  Having built several variants, the
best approach is to solder the feedline to the element.
(I still use screws on the driven element, but for
mechanical support only.)  The stub length will vary
somewhat depending where you solder it:  if you
solder it to the opposite side of the driven element
from the coax then the element itself contributes
part of the stub length.  I still end up trimming
the driven element to get a match with the stub in
place, but the SWR shouldn't be that bad to start.

I just ran an EZNEC model of the beta match and came up
with a 6" stub spaced 1" and a driven element length of
37 1/4".

Then I went out and measured the one I just used today
and it was 36.5" with a 2.5" stub (though the stub has
been bent enough times that the spacing isn't exactly
even - I think I pinched it around to get a match.)

But even in the worst case your SWR should be better
than, say, 6 : 1.  With the stub removed the model gives
an SWR of just over 3 : 1.

If your SWR is 10 : 1 or worse, you probably have a
problem in the coax or a bad connection to the driven
element.

If you still can't find the problem, post back and we'll
try to track it down.  I haven't checked the SWR on my
beam for a while, but can do so if that would help.


Good luck!

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