"Larry" wrote in message
...
wrote in news:1174870222.221326.262340
@n59g2000hsh.googlegroups.com:
I'll be curious to see how this effects my HAM radio performance since
the original installation had coax as the feedline!
It won't help. Boats running low power (150W) and end-fed, untuned short
wire antennas have SUCKY signals, no matter what you do....especially on
the lower HF bands. A 55' backstay, between the insulators, is an
antenna at 8.5 Mhz, if you have a good ground on the tuner (1/4
wavelength) and a great 1/2 wavelength antenna at 17 Mhz, where it
doesn't even need a ground system to work against. At any other
frequency band, especially below 8 Mhz, the antenna is way too short to
fit the RF wave onto and the lower you go the worse it gets. Between 8.5
and 17 Mhz, the antenna is a complex impedance with a lot of inductive
reactance. The wave doesn't fit well the further away from 8.5 and 17
Mhz you get, killing its radiating potential.
Use 468/ft length = Mhz to determine what resonance is for your backstay.
It'll work fair there on 150W. Far away from there in frequency, it'll
suck, just like everyone else's. Hams use resonant antennas for a
reason...(c;
It might help, and shouldn't hurt. Larry, the reason ham use resonant
antennas is because they can. All things equal, a resonant antenna will
perform better, because there will be less loss in matching networks, ground
systems, feedline loss, etc.
HOWEVER, on a boat (on *my* boat, anyway), puttiing up, and keeping up, a
resonant antenna for all the frequencies I regularly use is too much of a
nuisance. I have an insulated backstay antenna, about 50 ft long, and a
pretty good ground system. I have an Icom AT-130 tuner right below the
backstay, with a short length of high-voltage (GTO-type) wire from the tuner
to the backstay. There is a wide copper strap from the ground system to the
tuner. The radio is an Icom 710-RT. This is a pretty standard
installation, and it works very well.
I'm sure it could work better, but it during last summer's race from San
Francisco to Hawaii, and our trip back, we were participating in a
marine-band net once or twice a day. VALIS (my boat) typically had one of
the best signals out there, on 6 and 8 MHz. Yesterday, I had re-installed
the radio after some maintenance, and my first radio-check contact was on 20
Meters with a ham in the Carribean -- I was in San Francisco. My point is
that it works well enough. Yeah, it isn't as good on the lower frequencies
as a full-length antenna would be. A different installation may be better,
but so would a tri-band yagi at the top of my mast. I'm happy to stick with
what I have.
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.
(Larry, and anyone else interested -- I will be asking a question about
Raymarine vs Furuno nav systems, and would appreciate some advice. I will
start a new thread over in rec.boats.electronics so I don't hijack this
one.)
- Paul (wb6cxc)
- S/V VALIS -- PSC44 #16 -- Sausalito, California
-
www.sailvalis.com