Fred W4JLE wrote:
Can you give us a magnitude, for example for a g5rv on 3.8 MHz fed with 50
feet of 50 ohm coax. Is it .5 or 5 dB?
Sorry again, I can't help you much. There are a number of tuner
topologies in use (L, pi, high and lowpass tee, LCC, "ultimate
transmatch" to name a few), and the loss of one compared to another can
be quite different depending on the load impedance, even if all use the
same inductor(s).
The information you gave is enough to calculate the impedance seen by
the tuner, though, assuming your G5RV is 102 feet long and not one of
the countless variations. Assuming a height of 40 feet over average
ground, the impedance at 3.8 MHz seen at the input end of 50 feet of 50
ohm coax should be about 2.5 + j23 ohms (SWR ~ 27:1) if the coax is
RG-8, or about 4.1 + j22 ohms (SWR ~ 17:1) if the coax is RG-58. (The
SWR at the load will be greater than 60:1. You should maybe be worrying
more about feedline loss than tuner loss.) I didn't take into account
the effect of feedline radiation, which I'll assume is suppressed with a
balun.
Or we can be more specific and say we want to match
a 50 Ohm source to R 5000 -J720 using an MFJ989C. What loss is to be
expected?
Using the Z you specified or the Z I calculated, all that's necessary
then is to obtain a schematic of the tuner and information about the
physical construction of the inductor(s), calculate what settings will
achieve a match, and find out which of these settings you're using. Then
calculate the coil currents and from that the loss. I don't have a
schematic of the tuner nor a physical tuner -- I haven't used one for 30
years or more except for a small homebrew tuner for correcting small
mismatches to keep my QRP Field Day rig happy. So I don't have a clue.
Perhaps someone else who has the tuner, the math ability, the patience,
and the time will be willing to help you.
Granted there are cases of attempting to load a short (with respect to
frequency) antenna that has extremely low values of R can cause large
losses. I would think that a general statement could be made about antennas
that are 1/2 wavelength or longer at the operating frequency and the
magnitude of losses one could expect from the typical tuner.
I think you can say that if the SWR is no greater than 2:1 or maybe 3:1,
the loss in a properly designed tuner should be pretty low. Outside that
range, like your 17 - 27 SWR example, I wouldn't speculate.
Is there no way to quantify the losses, no rule of thumb, should we all
throw our tuners away because they may have unpredictable horrendous losses?
Hm, are those the only two options? Why not use the tuner, make
contacts, and be happy? And if the tuner has a horrendous loss, why will
throwing it away help? You'll still be better off with it than without it.
What steps should be taken by the average amateur to obviate these losses.
Adjust the feedline length for each band to present a moderate impedance
to the tuner. Cecil has posted quite a bit about this in the past. And
use a tuner with the largest coil(s) possible. You could set up jumpers
so you can change the topology of the tuner to pi, L, tee, etc. On each
band, try each configuration and, among those which effect a match,
choose the one that gives you the greatest field strength from your
antenna or the highest current to it.
Of course, you could do what I do -- I settle for other than all-band
operation, and make my antennas resonant or nearly resonant. When
matching is required, I do it at the feedpoint with components I've
characterized -- I know the antenna Z and the inductor Qs, so I can
calculate the loss. But most people don't want to do this, and happily
put up with whatever losses they get in exchange. Or you can get
something like the B&W terminated folded dipole, and put the losses at
the antenna instead of the tuner.
I am at a loss to understand how bad the losses are that you and Cecil refer
to. Can you lead me from the darkness Sir!.
Nope, sorry, I can't. What's the approximate value of resistors? I can't
answer that one, either.
Roy Lewallen, W7EL
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