From what I understand, a tuner's VSWR reading is for the link between the tuner and amp. Is the reflected power figure a measurement of reflected power from the antenna to the tuner? Or is it what the tuner is reflecting back to the amp?
SWR meters in tuners are designed to read the SWR between the radio and the tuner,
because that is where you are trying to minimize it.
If you want to see the SWR on the feedline itself, you can install the tuner backwards,
put an external meter after the tuner, or just switch the tuner to BYPASS and feed the
antenna directly.
The SWR at the end of the feedline is useful to know when setting up an antenna, but
not necessarily something to check every time you readjust the tuner.
Meter accuracy is going to limit the resolution to which you can read high SWR: there may
be some that can measure a 10 : 1 SWR accurately, but it gets much more difficult
beyond that.
As to your statement about ladder line being operated with terrible SWR but little loss.. I never understand that concept until just now after making up an excel spreadsheet and seeing for myself. Wow.
The VK1OD transmission line loss calculator does this for you.
So now Im off so search for VSWR data for all bands on the 102' and 135' all band doublets.
You really want
impedance data, not just SWR.
Something like this: http://vk1od.net/antenna/G5RV/
Last ?: For a coax-fed antenna that is mistuned for TX operation to the figure of 90:1 at the antenna using coax with 1.6dB of line loss, how bad will it be for receiving signals? I understand Ill see at least 1.6bB of loss, but am wondering about mismatching in regards to RX operation.
In practice losses are generally similar for RX and TX, but the mechanisms are somewhat
different. On receive we can provide a 50 ohm receiver input so the SWR is 1 : 1 on the
coax. This holds the coax losses to 1.6dB in your example. But let us assume that the
antenna is 5000 ohms (SWR = 100 : 1) and, for the sake of illustration, there is a very short
length of 5000 ohm feedline between the antenna and the coax connection. On this piece
of imaginary feedline the SWR is again 100 : 1 because it sees a 50 ohm load. The power
from the antenna flows though this 5000 ohm feedline and hits the 50 ohm of the coax.
What happens? Most of the power is reflected back up to the antenna. Since the antenna
matches our 5000 ohm feedline, that reflected power is all dissipated in the load (re-radiated
from the antenna, actually.) We can let the length of our 5000 ohm transmission line go
to zero, and assign no loss to it, and have the same result in the real world.
[Brief pause for math: return loss in dB is given by -20 * log ( (VSWR - 1) / (VSWR + 1 ) ).
For an SWR of 100 : 1, the return loss is 0.17dB. That means that the reflected wave is 0.17dB
weaker than the forward wave. The amount power delivered to the 50 ohm load is the reciprocal
of that, or about 5.75dB down from the incident wave.] So the loss due to antenna mismatch
is about 6dB.]
You then add the coax loss (1.6dB in your example) and find that on receive signals are
over 7dB weaker due to the antenna mismatch. That compares with ~13dB loss on
transmit: not the same, but higher than one might expect if you only looked at the feedline
losses when the receiver input stage perfectly matched the transmission line impedance.
Assuming, of course, that you are not using a tuner in the shack, as that would change
the SWR on the coax in receive mode.
Let me add a disclaimer: this is the first time I've run such a calculation, and while it seems
like it is based on solid transmission line theory, it wouldn't surprise me at all if someone pokes
a hole in it. But it is an excellent question, and I've never seen a good explanation of it.
