Towertalk
[Top] [All Lists]

Re: [TowerTalk] Wire antenna help needed

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Wire antenna help needed
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Wed, 22 Mar 2017 10:46:53 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On Wed,3/22/2017 7:14 AM, K7LXC--- via TowerTalk wrote:
  I need some feedback on a multiband wire antenna.
As Howie has clearly stated, any OCF antenna will put a LOT of common 
mode on the transmission line. This makes it a sitting duck for noise 
pickup. It is not practical to choke an open wire line -- it must be 
choked after you're in coax, and the severe imbalance places heavy 
stress on a choke. Which is why Howie is using two for 500W.
There's another issue that N6BV (retired ARRL Antenna Book editor and 
author of HFTA) pointed out in a piece he wrote for QST several years 
ago. There can be very high DIFFERENTIAL MODE loss (that is, inside the 
coax) if the standing wave pattern of an antenna that's FAR from 
resonance causes a current maxima at the location of a  coax choke wound 
around ferrite cores. The same would be true for a bifilar choke 
connected as a transmission line. No question that the loss when running 
an amp could fry the choke (or melt the coax).
And, as N6BV observed, the additional loss due to mismatch can be quite 
high in the coax.  Remember that SWR is determined by the match between 
the antenna and the line, and this is what determines loss in the line. 
An antenna tuner does not change that -- it only matches the line to the 
transmitter so that the transmitter can put power into the line. In 
Dean's extreme example of a resonant antenna loaded on its second 
harmonic, much less than 100W got to the antenna with 1.5kW going into 
the line.
Bottom line -- unless you're in the middle of nowhere (NO neighbors, no 
cell sites, repeater systems, etc), ANY antenna feedline is going to 
pick up noise unless it's choked, and it isn't practical to choke open 
wire line. The antenna will also be hearing that noise, so the 
contribution of the feedline is likely in the range of 2-6 dB. Not a 
deal-killer, but 6 dB can cover up a lot of weak signals that you want 
to work. Like the two DXpeditions in AF that I managed to put in the log 
last week on 160 and 80M. For three of the four QSOs, with 2 dB more 
noise, I wouldn't have heard them well enough to work them.
Another point - in Howie's example, with the choke after conversion to 
coax, the part of the feedline between the antenna and the choke is 
reallty part of the antenna and is picking up vertically polarized 
signals and noise. That choke forces a current minima where it's placed, 
but we're dealing with RF here, not DC, so voltage and current vary 
along that line just like any other antenna, and the length for common 
mode is for VF=1.
73, Jim K9YC


_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk

<Prev in Thread] Current Thread [Next in Thread>