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+--- Thread: Low Z On Transmission Line - What Results? (/thread-663.html)
Low Z On Transmission Line - What Results? - VK4CCV - 14-10-2018
G'Day To All,
My scenario is as follows:
The back yard only allows me to string out a 88' Dipole.
The dipole centre is 33' while the ends come down to 20'.
From the feed-point is a 33' length of 450 Twin Lead to ground level where it enters a 9:1 Balun, then 40' of 50 ohm low loss back to a tuner.
I particularly like operating on 160 & 80.
At 160, the Impedance at the balun will be 270 ohms, not a big drama, but on 80 the impedance falls to 50 ohms.
Will the signal on 80 be greatly affected do you believe?
Pete VK4CCV
RE: Low Z On Transmission Line - What Results? - VK4ADC - 14-10-2018
Peter
There will be losses but what will happen is the ALC will drop back the RF power in the rig due to internal SWR sensing the high inherent SWR. An in-line tuner can negate the mismatch but it introduces losses too.
Modify the 9:1 balun to optionally give you 1:1 and use a switch / relay / jumpers / cliplead arrangement to allow you to change the ratio depending on which band is is use. That is assuming that "ground level" is actually reachable if doing it manually.
Typical 9:1 balun wiring is like at http://www.spirat.com.au/vk5zvs/pic31.htm but add another 2 x 3-turn windings so that you can have a 1:1 output from the same balun core, centre joint of the 2 x 3T to ground....
Switch both feeder legs from the 9:1 to 1:1 outputs - use a DPDT relay to achieve it remotely.
You might even find that the variability of the two impedance selections will let you use the antenna on other bands as well.
RE: Low Z On Transmission Line - What Results? - VK4CCV - 14-10-2018
Hey Doug,
Thanks for the response, much appreciated.
Yea, other bands are of interest to. To tell you the truth, the whole HF bands are used but I like to rag-chew on 160 & 80.
You have given me some good ideas to ponder upon.
Thanks again Doug.
Pete