Paul wrote:
SNIP
As has been mentioned, using coax from the tuner to the end of the backstay
has two disadvantages: It adds shunt capacitance, and some loss, which can
makes the tuner's job more difficult. Also, it may arc through from the
center conductor to the shield, during operation at certain frequencies
where the voltage can rise to very large values.
SNIP
- Paul (wb6cxc)
- S/V VALIS -- PSC44 #16 -- Sausalito, California
- www.sailvalis.com
Hello Paul,
Part of the problem in analyzing a run of coax between the tuner and the bottom of the backstay is agreeing on what the alternative is. If the alternative is having the tuner right at the base of the backstay, then that will usually give the best results. (But see below)
But if the tuner is, say eight (or more) feet away from the bottom of the backstay and the run is more or less along the waterline, then that's not going to be a very attractive alternative. On frequencies where that length constitutes a current node, (especially at higher frequencies where the antenna might be a short, automotive-type whip) that length is an important part of the radiating system. But its radiation is essentially into the water where it does no good.
A variation on that is where the connection between the tuner and the "ground" is several feet long: a not unusual arrangement. In that case, the antenna actually begins at the ground system and the ground wire running to the tuner is a full, radiating part of the antenna!
So the rules are not just to avoid coax between the tuner and the backstay, but to install the tuner as close to the backstay as possible and to install the ground system as close as possible to the tuner. Fortunately, these rules are tempered with the knowledge that many installations work acceptably despite their departures from the ideal. ;-)
I would second Bruce's recommendations, and also point out that on the Chesapeake and its tributaries as well as on the Great Lakes, you will need to provide lots of area for your RF ground because of the water's lack of salinity.
73,
Chuck
NT3G
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