Posted by Brian Guptil (sailordude@…>)
HF Antennas
We keep hearing “size matters”, and those sympathetic may say “size doesn’t matter.” Well in antennas “length matters,” epically if you are trying to operate without an antenna coupler. We are discussing modifications to a dipole antenna. This antenna is a 1/2 wavelength center fed configuration. At the 80 Meter band, that antenna is 40 meters long. As you change the frequency, the antenna becomes more (out of tune) and the SWR starts climbing, and at some point the transmitter will shut down, go into power limit mode or overheat the final amplifier’s). You can cheat this by having multiple length raiding elements, but this gets a little clumsy as you stretch 5, 10 and 20 Meters elements on both sides of the center insulator.
You can eliminate half the length of wire antenna by substituting the non-radiating side of the antenna with “earth.” If your antenna is near 1/4 wave length and the connection to earth is very robust, you probably will have a matched antenna. Now, if you are going to operate on other then “tuned length” you need to fool the transmitter into thinking it is feeding a 52 ohm resistive (matched) load and the antenna coupler comes into play. The coupler has a tunable inductor/capaciter combination filter that (effective) adds or in some cases, subtracts length to the antenna as far as the transmitter is concerned. On both the transmit and receive functions, an antenna less then 1/4 wave does not have the same ability to transfer RF energy to or from the outside world. The pseudo antenna length is actually, inside the antenna coupler BOX, not exposed to the sky.
On the receive side, there are two measures, signal strength and signal to noise ratio (s/n). Noise is made up of electrical interference external to the receiver, noise generated in the receiver, and atmospheric noise one hears between channels on the AM and FM radios etc, now thought to be echo’s from the Big Bang, so say scientists in radio astronomy. You want to amplify the signal, not the noise. Best done with good antenna rather then a short antenna with an amplifier.
S/V Cayenne has a coupler, stbd side hull adjacent to the mizzen, with an insulator feed thru to the deck. I can hang wire, vertically with flag halyards, or an inverted “L” shape, hoisted on the mizzen and then stretched across to the main, both using flag halyards and appropriate insulators. Ground plain is a 2 in wide copper strap run all the way around the inside of the hull at about a foot below the water line. Too bad, lightning took out the IC M-802.
The Best antenna I worked with (other then very long wires) was a 27ft fiberglass whip. It’s advantage was ease of matching (low Q, broad tuning range) to coupler. But on self-righting motor surf boats, DX’ing is not the purpose, and having a forgiving antenna/coupler combination was very important. Remember, a sailboat is a dynamic environment where the antenna proximity to the water changes as the boat rocks and roles.
Oh, and stainless is far from the best wire for an antenna. But then compromise is everywhere!!!
Brian
On 4/8/08 3:33 AM, “michel.capel” <michel.capel@…> wrote:
\
George,
I’m far from a radio expert, but this is what I read:
An option I heard of often, is using one of the (stainless)
backstays as a short wave antenna. Put isolators in it and mount the
balun (?) at the top, with the connection wire going into the
masthead and then down through the mast. In that way you can always
use the antenna, where ever the backstay is positioned. Another
(electronically better solution, I understand), is to mount the
balun at the foot of the wire and have a removeable connection wire
to the receiver.
Another option, as fas as I know, would be a short, rigid (2’)
active short wave antenna with tuner. As far as I understand, these
modern antennas are better than the old isolated wire. You can mount
the active antenna on the masthead or on the aft rail. Perhaps even
halfway up against the front of the mizzen mast. I know this cannot
be done with aluminum masts (it hinders reception/transmission) but
perhaps a radio frequency expert could say it a CF mast will be a
hindrance too. I would be very interested in this answer too, also
with respect to VHF reception and transmission.
I take it that counterpoise is a term for electromagnetic ground? If
that’s correct then, as a ground plate, you can use special bronze
plates that you mount on the outside of the hull. They have a
special ‘puckered’ surface to maximize the area of bronze in contact
with water. They are about 6" x 2". On the inside, you connect your
groundwire to the plate.
Hope someone corrects me it I made an error in some of this.
Michel
— In FreedomOwnersGroup@yahoogroups.com mailto:FreedomOwnersGroup%40yahoogroups.com , “George Huffman”
<thatboatguy2@…> wrote:
Hi,
I would like to renew the discussion on hi frequency radio
antennas.
I recently inherited an ICOM M 745 Transceiver and ICOM tuner. It
was
part of my compensation on the latest integrated electronics
install
that I did. So it looks like Marquesa will have ears after all.
I’m
wondering about what other F 40 owners are using for antennas and
counter poise? Interestingly enough one of the little radio guide
books I have shows a line drawing of an F 40 to illustrate the use
of
a dipole antenna hoisted aloft. I would much prefer a permanently
installed antenna that we could use in seaway. I think what we
arrived at last time around was that a whip antenna was going to
have
to suffice but I’d still like to hear what’s actually being used
out
there.
Thanks
George
Looks like there is far from agreement on what’s best concerning the ground plane. I’m going to rig two radios in the near future. One is an ICOM 802 and that’s a paid job in which I must stick to the manufactures installation instructions. But on our boat I’ll experiment. The 802 is in Mandeville LA. and we are in Annapolis so maybe we will try and talk to one another! Also the 802 I’m setting up with a PacTor III modem and that should be interesting. I had everything mounted up before I left and when I fly back down in late May I’ll do all the wiring. If the attachment doesn’t work I’ll send a link next time. Thanks Steve,We are much alike you and I. My first cruising