Backstay aerial

[Graham_Wright]

I have two backstay insulators I wish to use for a receive only (at least initially) aerial.

What is the optimum spacing and height off the deck. The boat is a Countess 33 centre cockpit and the mast is 43 feet high.

Welcome any advice.

Graham Wright

<hr width=100% size=1>GBW

 

[snowleopard]

basically grab as much as you can. it is a british convention to put the lower insulator above arm's reach to prevent burns, others don't bother about that. in any case the lead up to the insulated section acts as part of the antenna.

when you come to use the aerial for transmitting you'll need to add a very effective earth and an antenna tuner joined to it with wide copper strip.

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[Guest]

Yes grab what you can for receive only and dont bother with the bottom insulatr for receive only, I dont use a bottom one even tho I transmit. If you intend to upgrade later a transmit as well, avoid multiples of 1/4 wave on the frequencies you will use.

<hr width=100% size=1>Barry (yacht Mithril)

 

[IanBBA]

No, no; well almost! Odd 1/4 wavelengths are good; even multiples are very bad.

Radiating elements of 1/4, 3/4, 5/4 ... etc offer a low 'impedance' not far away from the 50 ohms your transmitter requires and are easily matched in an ATU (antenna tuning unit).

Radiating elements of 2/4, 4/4, 6/4 ... etc offer a very high impedance and although a good ATU will match this to 50ohms, very high impedance = very high voltages (P=V^2/R).

A 1/2 (2/4) antenna may typically have an 'impedance' in excess of 2,000 ohms. Your 100W transmitter will therefore produce 100=V^2/2000; V=~450Vrms at the feedpoint (backstay bottom insulator). The same transmitter power into a 1/4 antenna of ~50 ohms will give a feed voltage of ~100Vrms; far safer and puts less stress on insulators and the ATU.

[Fellow techies will note I have ignores reactive impedances of the 1/2 wave in this simplistic example or Vpk complications for SSB operation.]

Calculating the 1/4 length is easy; length (m) = 71/f, where f is the transmit frequency in megahertz.

Another complication now enters the theory; ‘harmonic’ relationships of the HF marine bands. If you calculate a ¼ wavelength antenna for the 4 MHz band, this would be ~17.8m in length. If you used this antenna on the 8 MHz band, where the ¼ length is 8.9m, your 17.8m antenna is actually 2x 8.9 or a ½ wavelength long and therefore high impedance at 8 MHz! Similar problems arise with 6 & 12, 8 & 16 …etc, MHz Bands.

After all this theory, the best practical advice is to make your backstay as long as possible and if you use an auto-tuner that struggles, shorted the antenna! On receive, none of this is an issue. One final point, earth the mast; 100W in your back stay will induce enough power in an unearthed mast for a potential burn.

Good luck, Ian.



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