HF/SSB antenna inside Dyneema lifelines

[Jammer]

Originally Posted by j.g.evans

It sounds like you will have a good antenna to talk to fish and god. You need a lower take off angle.

I like high takeoff angles.


I have never spoken with God via shortwave but I find that NVIS techniques are real and work extremely well at the distances and frequencies I care about, to wit 0-200 miles on 40 and 80 meters.

[/QUOTE]

[sailcub]

I realize everyone is always an expert on any and all things boat related....the GAM performed flawlessly hooked to a ICOM 802, tuner and a grounding plate...we never had an issue either on HAM or SBB channels...

[Jammer]

Originally Posted by sailcub

I realize everyone is always an expert on any and all things boat related....the GAM performed flawlessly hooked to a ICOM 802, tuner and a grounding plate...we never had an issue either on HAM or SBB channels...

I don't doubt that it worked great for you.

I am skeptical that it worked better than just connecting a wire to your backstay chainplate bolt. This works OK. As noted upthread, the main problem with it is that, without an upper backstay insulator, it lights up the whole rig, which can lead to unpredictable problems on certain frequencies or in certain directions, or both. The GAM largely does the same thing because of the capacitive coupling between the GAM and the backstay.

I am skeptical that it would work in fresh water, because a ground plate is not useful in fresh water.

[justlearnin]

What about the shock hazard of having so much rf radiation at such a low altitude.

[Jammer]

Originally Posted by justlearnin

What about the shock hazard of having so much rf radiation at such a low altitude.

https://en.wikipedia.org/wiki/Near_and_far_field


It really doesn't matter much because with any HF transmission, regardless of frequency or antenna type, you are inside the nearfield on a sailboat. You can't get a wavelength (i.e. 40 meters or 120 feet) away from the backstay.


In the lower HF bands at 100 watts there isn't much of a hazard unless you are in contact with a conductor or nearly so (less than an inch). Your VHF handheld is, by the numbers, more hazardous to your health.


The California Highway Patrol used Motorola Maratrac radios on their cars, at over 100 watts output, with an antenna in the middle of the trunk lid, for years, in VHF low -- a far more hazardous band for human health than 40 meters. That went on for decades. There are quite a lot of chppies and always have been, and there is no pattern of illness or injury in this population.


I've spent a lifetime around transmitters and antennas, and the ones I am afraid are the VHF and UHF ones that put out more than 1000 watts, and the parabolic antennas with >20 dBd gain on any band.

[team karst]

Originally Posted by Jammer

https://en.wikipedia.org/wiki/Near_and_far_field



I've spent a lifetime around transmitters and antennas, and the ones I am afraid are the VHF and UHF ones that put out more than 1000 watts, and the parabolic antennas with >20 dBd gain on any band.

I recall that the body tends to be resonant at near 2meters. So, no fiddling with your EME antenna in front of the reflector(s) during 1kW xmit!

[Jammer]

Here's a sketch.

Electrically, on each side of the boat there's a two-band fan dipole that is electrically lengthened to be resonant by using loading coils. The lower band of the fan is on the upper lifeline (because it is somewhat longer) and is roughly center loaded. The high band goes on the lower lifeline and is off-center fed, that is, with the same physical feedpoint as the lower band, but the element that goes forward is electrically shorter, by using a smaller coil, and by making the physical wire somewhat shorter. The aft-going element goes across the boarding gate, and rather than have a jumper that gets in the way and has to be disconnected to use the boarding gate, the wire goes down to deck level, across the gate, and back up.

The antenna on one side of the boat is tuned for 3800-4050 kHz and 6200-6320 kHz covering the amateur 80 meter band and portions of the 4 kHz and 6 kHz marine bands.

The antenna on the other side of the boat is tuned for 7175-7300 and 8200-8300 covering the amateur 40 meter band and the 8 kHz marine band.

I will have to use a VNA to get everything tuned. I have experimented before with offset feed, center loaded fan antennas, and there is always a certain amount of cross-tuning that occurs -- when adjusting one choke, other bands are affected in some way. The VNA helps make the tuning process quick and accurate even with these effects. The 80m band is the widest and there will be some SWR increase at the edges which may lead to some power foldback in the transmitter. Other bands should have good SWR across the target frequencies.

Antenna elements are 14 gauge stranded THHN run through the center of the Dyneema lifelines wherever possible. The feedpoint on each side of the boat is through a current choke made with 14 turns of THHN around a 2.4 inch toroid of #31 material. The choke is fed from RG58. The two RG58 feedlines are joined in a T at the transceiver.

I expect that the outcome of all this is that I will be able to transmit on all the frequencies used for regional nets, all the frequencies used between fellow cruising boats at distances beyond VHF but less than about 200 miles, and at several of the most important DSC and voice distress signalling channels used by the USCG at these distances.

No tuner, antenna switching, or ground plate should be necessary.

[stormalong]

Never mind the antenna. What is that giant hole???