Hello Fred.
12 Apr 97 15:21, Fred Mckenzie wrote to Roy Witt:
 FM> In a message dated 04-07-97 ROY WITT wrote to JEFF EDMONSON:
 FM>
 RW>> If I tune an antenna with the swr bridge at the antenna and then
 RW>> place the swr bridge at the other end of the coax, there's a
 RW>> difference. 1.2:1 at the antenna, 1.9:1 at the tx. No amplifier,
 RW>> No antenna tuner, no etc.. Now what do I tune to get 1.5:1 or
 RW>> less at the tx?
 FM>
 FM>   Roy-
 FM>      Could it be that you tuned the antenna with the bridge at the
 FM> antenna end of a test cable?
I don't have a bridge, per sey.  It's a Bird 43 wattmeter, which I have 5, 
25, 50 and 100 watt, VHF/UHF slugs for.  A little too large to connect to a 
VHF/UHF antenna.  I use a piece of (RG8A) coax cable with teflon connectors, 
cut to an uneven number of multiple half-wave lengths.  In this case, it's a 
UHF antenna and the test coax is 7 half-wave lengths long.
(.66 x 246/F(460MHz)=4.235" x 7 = 29.65" long, connector end to connector 
d.
I haven't cut the test coax for this frequency, as I figure the one I have 
cut for 445MHz (30.65") should be close enough.  Also, my antennas don't 
mount with the elements next to the mounting pipe.  These antennas are 
mounted with a part of the boom extended about 8" to the rear of the 
reflector element.
 FM>   With VHF/UHF antennas, I would expect the presence of the meter
 FM> and/or your hands, would affect tuning of the antenna.  When you
 FM> remove the bridge from the vicinity of the antenna, the antenna's SWR
 FM> may actually be higher.
 Right you are.  You must make your adjustments and then move yourself back 
before you test.
 FM>      If you replace the antenna with a good dummy load, is the SWR at
 FM> the transmitter end of the cable still 1.9:1?  That would suggest a
 FM> conflict between the design of the SWR bridge and the characteristic
 FM> impedance of the cable.
The wattmeter and test coax are 1.0:1 into either one of two oil filled, 
50ohm dummy loads.  With the site coax between the transmitter and wattmeter 
into the dummy load, the swr goes up.  With the antenna connected, it goes 
higher.
 FM>   Assuming a good bridge and the correct cable impedance, I would
 FM> expect a long run of cable to appear to lower SWR due to losses, not
 FM> raise it!
Except for a few long hard-line cable runs to the top of a 200ft tower, most 
of the antennas I sell are used near the 50 foot or lower level and are fed 
with 9913 or RG8A.
 FM>      What if you separated the antenna from the bridge by a convenient
 FM> length of cable, like one wavelength?  That should be close enough to
 FM> avoid cable losses, yet not be so close as to affect tuning.
I suspect the site cable used has been contaminated by weather as the antenna 
checks out perfectly under all my tests.  I've suggested the owner switch to 
a new piece of RG8A or 9913 cable and try again.
 Thanks for your input, Fred.  It's appreciated.
... My Twit Filter just put me on its Twit List!
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