Thread: NVIS and VHF?
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Old May 24th 11, 10:54 PM posted to rec.radio.amateur.antenna
Dave Platt Dave Platt is offline
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First recorded activity by RadioBanter: Jul 2006
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Default NVIS and VHF?

In article ,
'Captain' Kirk DeHaan wrote:

All of this leads me to think that something else is wrong. I can hit a
repeater 20 miles from me with 5 Watts and its antenna is on a 300 ft
tower. My antenna (half wave end-fed dipole) is on a 20 ft mast.


There may well be something wrong with the antenna. I don't have an SWR
meter handy but will try and borrow one from a club member. The J-pole
was beat around quite a bit over the years but since the radio has a
limiting circuit if the SWR is too far out of whack I am assuming it is
fairly close. It will transmit of full power.


One of the things to know about the American Legion J-pole (and most
vaguely-similar J-poles) is that it has no feedline or mount-point
isolation to speak of. The outside of the feedline coax, and any pipe
or tower or other metal to which you strap the antenna, become part of
the antenna's ground system, and can radiate a significant amount of
RF. This can interact with the RF coming from the main radiator, and
significantly alter the antenna's pattern... it no longer behaves like
an idealized half-wave vertical radiator.

In most installations this doesn't seem to matter all that much... but
it's possible that you've got some sort of weird interaction (feedline
emissions, reflections from the ground or nearby metal, etc.) which is
creating a null in the antenna's pattern in just the wrong direction.
If this is an issue (not all that likely but perhaps possible) you
could insulate the antenna from the mounting mask, and install a choke
(e.g. a few ferrite cores) on the feedline just below the antenna.

Also, the American Legion J-poles are subject to a couple of forms of
mechanical/electrical failure after a few years. The plastic
insulator which spaces apart the radiator and matching arm can degrade
(UV from sunlight is the main offendor, I believe) and crack, allowing
the two arms to change spacing. More subtly, the connection between
the two arms and the metal base can degrade... it's aluminum-to-
aluminum using a set-screw, and the connection can loosen and corrode.

It might be worth checking the spacer (replace if necessary), and
checking and tuning up the set-screw connections (squirt with Liquid
Wrench if necessary, loosen and remove, take out the rods, clean the
ends of the rods and the mounting holes in the place, dab with NoAlOx
or a similar corrosion-blocker, reassemble and tighten).

I'm using 75' of
professionally assembled RG8X which is required to get from the radio to
the antenna.


That's probably costing you about half of your power (assuming a
typical RG8X-type coax). Some lower-loss types (e.g. LMR-240 are
better than this.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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