Thursday, November 21, 2013

SWR - How High is Too High?

To repeat something I've said before, I am more interested in an antenna's pattern than its match. That does not mean that the match is unimportant. What I am looking for in an antenna is one with the pattern that suits my operating interests (primarily DX) and a match that does not get in the way of the pattern being attainable in practice.

The match -- and therefore SWR -- has two major criteria:
  • Transmission line and conductor loss -- Additional loss due to mismatch.
  • Rig compatibility -- Tolerance to mismatch, which may result in folded-back power.
Let's do this with an actual example from my station. My multi-band inverted vee is cut for 30, 20, 17 and 15 meters. Its SWR is low on all these bands. The antenna also works on 10 and 6 meters, but with higher SWR.

With the developing double peak in the current sunspot cycle there are daily openings on 10 meters. I deliberately didn't bother to add 10 meters to the inverted vee because I hadn't really expected the openings on 10 to be so good. My assumption was that the other multi-band dipole -- TH1vn -- would be sufficient, even though it has sharp side nulls. With these great openings I really want to fill in those nulls with the inverted vee since it is oriented nearly perpendicular to the TH1vn. Plus it is higher (14.2 meters apex).

The inverted vee hears well. On transmit the SWR is 2.5 at 28.000 MHz. That is generally considered high for a modern, no-tune transmitter. My KX3 QRP rig has no problem putting out 10 watts (full power on this band) into that high-SWR load.

Since the rig doesn't complain I have no reason to complain. That's all there is to that. I trust Elecraft to properly engineer their rigs. The transmitter doesn't fold back with such a load, and it hasn't yet "fried" after many QSOs with this antenna on 10 meters.

That's one question answered: the rig doesn't complain so the SWR, though fairly high, is acceptable.

Now on to the transmission line loss due to the SWR. The run of coax is about 25 meters of RG-213/U. The common-mode choke is made from RG-58 let's round up to 30 meters (100') equivalent of RG-213/U.

The matched loss of this transmission line is approximately 1 db/100'. Plugging the numbers into an online calculator I get the following results:
  • Feed point SWR is 3.4
  • Total loss is 1.7 db
The additional transmission line loss due to the SWR is less than 1 db. That's a small sacrifice when you consider that the side null of the other antenna is well over 10 db.

The analysis is just as simple as shown above. Rig happy? Check! Acceptable transmission line loss? Check! We're good to go. If all works as it should any SWR isn't too high.

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