Topband: Inverted L Matching

[Pete Smith]

k6se@juno.com wrote:
>>
>>Re Jack, W4NF's inverted vee:
>>If the transmitter pi-network output can tune into a 75-ohm load, then an
>>easy method to lower the SWR at the transmitter would be to use a
>>1/4-wave length of 50-ohm line between the antenna feedpoint and the
>>transmitter.  Such an arrangement would yield a 75-ohm SWR at the
>>transmitter of about 1.3:1 at resonance and the 2:1 SWR bandwidth  would
>>be from about 1.800 MHz to 1.870 MHz.

I'm not sure all this matters much at 160 meters.  Feedline loss from a 2:1
SWR probably isn't worth the trouble of setting up a matching network at
the antenna, unless you have a tremendously long run of high-loss coax.
For example, with 100 ft of RG-213 and a 25-ohm load, for an SWR of 2:1 at
resonance, the total loss in the coax is 0.276 dB, of which mismatch loss
contributes 0.038 dB.  Your potential loss savings from a matching network
at the antenna amount to something less than 1 percent of your power,
before you figure in the losses in the network.

Both SWR loss and total loss increase some with departures from the
resonant frequency, but that pretty much happens even if the antenna starts
out matched.

By the way, if you want to work through these figures yourself, I generated
them using TLA, which comes with the ARRL Antenna Book.

73, Pete N4ZR


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