Monday, July 18, 2011

Parasitic losses

If you have an SSB, you are probably like most boats - you have an insulated backstay for an antenna.  And because that insulated backstay is not a quarter wavelength at the frequency you are transmitting (I am assuming here...), you need some kind of antenna matching network between the transmitter, which has an output impedance of 50 ohms, and the antenna, which is probably considerably less.

The impedance transformer allows the transmitter to push power into a 50 ohm load, so all is good on that side, but what is happening on the output side of the antenna matcher?  Do you have coax running from the matcher to the antenna (you do if the antenna matcher is inside the transceiver).  Well, unless the insulated backstay is fortuitously that magical quarter wavelength, that coax is *not* the isolated feedline you might expect.  Both the shield and the center conductor are active parts of the antenna system.  It is as if you replaced the coax with twinlead.  Consequently, you cannot presume that the outside of the coax is inert as far as RF signal is concerned.  That is, you cannot use the normal rules for running the coax past metallic objects.

If the coax were being fed at its natural impedance, running it alongside of a metal object (say, a rain gutter, or more particularly, a backstay) would have little effect on things.  But when the coax is fed at the wrong impedance, the shield is active.  If you run it near metallic objects, they too will become part of the antenna system.  And if they are grounded, there is a good chance that their effect will be negative - that is, they will decrease the effective radiated power.

So let's go back to that coax feeding the insulated backstay.  Do you have it tightly fastened to the lower, uninsulated portion of the backstay like our PO did?  Not good.  Tho it will be following along the path of that portion of the stay, it should be held away from it in order to minimize coupling of the signal to ground.  On Eolian, it is.  Now. 

Using a trick I learned from Tom on s/v Warm Rain, I made standoffs to hold the coax away from the grounded portion of the backstay.  These standoffs are simply 2" pieces of 1/2" CPVC water pipe that I got at the hardware store.  I drilled the pipe with a 3/8" drill on 2" centers, and then cut the pieces apart thru the holes.

The standoffs are installed using snap-ties (use black ones - they will last a lot longer in the UV from the sun) going around both the coax and the stay, and passing thru the inside of the standoffs.  This is one of those cases where a picture is worth 1000 words.

73, Bob

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