Re: QDX with Elecraft T1


Roelof Bakker
 

Hello Wolfgang,

With due respect your example is a bit over the top.
Nobody with some basic antenna knowledge will feed a long wire with coax.
For reception only, yes, as on HF the additional loss of the coax will not degrade total
system performance.

- as in the calculation of SimSmith - the source is a generator with Zo=50 ohm impedance.

This is the case in the antenna modelling software, yet not the case in practice as the
transmitter is not a 50 ohm source.

On a related note, I have a 2 x 16 meter doublet, fed with 11 meter open wire feeder.
I have measured the SWR on the end of the open feeder and it ranges from 224 on 160 m to 6 on
40m and anything in between. The maximum feeder loss is 2 dB on 160 and 0.12 dB on 40 meter.
On average it is less than 1 dB, confirming that open wire feeders exhibit low loss.

Using a 4:1 Guanella balun, the SWR drops to less than 10 on all bands, except 160 and 30m.
From the balun I have 10 m Hyflexx 7 running to the shack. The SWR measured at the shack end
of the coax is very similar as measured directly at the balun.

I use a Z-100 plus automatic L-network tuner to match the antenna. It is interesting to note
that the measured antenna current in the open feeders are about the same whether the tuner is
used near the balun with 10m coax between tuner and transmitter or in the shack with 10 meter
coax between tuner and balun. In both cases the SWR at the transmiiter is less than 1.5.

One would expect that connecting 10m coax between balun and tuner should give increased loss,
which apparently does not happen. I can't explain this and if anyone can I will be much
pleased.

It should be noted that all feed-lines have loss. On 14 MHz 10 meter perfectly matched RG58C-U
has a loss of 0.5 dB.

An excellent loss line calculator can be found here:

https://owenduffy.net/calc/tl/tllc.php

As many of us are restricted to the use of wire antennas, an excellent source for information
is the book 'Successful Wire Antenna', an RSGB publication. Available from the RSGB Bookshop
and Amazon.

Rather than using a percentage of power loss, I prefer to use dB(m).

73,
Roelof, pa0rdt

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