On 27 mayo, 21:30, 'Captain' Kirk DeHaan
wrote:
John KD5YI said the following on 5/26/2011 8:56 PM:
On 5/24/2011 11:05 PM, 'Captain' Kirk DeHaan wrote:
Dave Platt said the following on 5/24/2011 2:54 PM:
In ,
'Captain' Kirk 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.
It is mounted on an angle bracket about 12" from a mast 20' above my
metal roof. The mast is isolated from roof but is not yet grounded.
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..
My spacer basically disintegrated. I replaced it with 1/2" pvc pipe. I
measured the spacing at the base and got it as close as possible.
I will check out the other connections. There is obvious aluminum oxide
on the assembly so it is very possible it is down between the parts.
I'll get up on my steep metal roof after the rains stop. :-)
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.
Thanks for the suggestions.
Hey, Kirk -
After reading Wim's response, I am more convinced than ever that you
have a problem with your antenna, coax, or transceiver.
What type of coax are you using? Could it have gotten water inside? Are
you using the coax that you used 10 years ago?
Try to think of other question similar to these and we'll get to the
bottom of this.
Cheers,
John
John,
I believe it is either the trees or the antenna. *The coax is new and
the transceiver worked fine last time I used it. *Also works fine with
this setup getting into the repeater on Schweitzer , 145.230, with 5
watts and the signal report from a gentleman I spoke to was 59. *That is
a clear shot and a few miles closer. *Although the profiles show there
should be a clear shot to the .780 repeater I can gurantee there is a
wall of trees on the first little ridge on the profile. *I found that my
club, which I've only been with for a month, has an SWR meter and an
antenna analyzer. *I hope to borrow them to see what's up. *In the
meantime I will pull the J-pole down and clean it up as suggested.
There is considerable aluminum oxide on the outside of the parts so
there may be some between the base and uprights. *When I put it back up
I will move it 180 degrees from it current location. *That will only
move it about 2' but who knows.
--
'Captain' Kirk DeHaan
N6SXR
"Beam me up Scotty, there's no intelligent life here"
Hello Kirk,
If you have access to both ends of the cable and have access to some
VSWR measurement equipment, you may measure following things (and post
them overhere):
VSWR of cable when terminated with a dummy load
VSWR of cable when open
VSWR of cable when short-circuited
VSWR of cable with J-pole antenna
Now reduce the output power of your transceiver to avoid damage
VSWR when nothing connected to the output of the VSWR meter
VSWR when output of VSWR meter is shorted.
Make sure to check the reference (forward) reading after every change
in load, as this may change due to varying load as seen by the
transceiver. You may also do these measurements with the antenna
analyser (preferably both antenna analyser and VSWR meter).
Regarding the trees, it looks dense. Sadly to mention, if it are the
trees, you have to live with this as tree attenuation at 2m is
relatively low and first fresnel zone is relatively large at 160m.
You need to remove lots of them to get remarkable improvement and you
mentioned that you couldn't remove lots of them because of financial
reasons.
With kind regards,
Wim
PA3DJS
www.tetech.nl
Don't forget to instruct your racing pigeon to remove abc before
letting him cross the Ocean.