...what is the advantage of having a remote antenna tuner? If I am using no more than abt 15-20 ft of coax feed line, is a tuner on my desktop not just as good?
It might be pretty close... or it might not. It depends on the impedance at the antenna and the loss in the coax.
Here is an example: I modeled a G5RV Jr. (based on Lou Varney's original design resonated at twice the frequency.)
Using the twinlead matching section the SWR on 20m is about 6.5 : 1 (it varies across the band.) Feeding this
through 20' of RG-58 coax results in a loss in the coax of about 1.2dB. That means you lose about 1/4 of your
power in the coax. With a tuner right at the antenna the coax losses are only 0.4 dB because the SWR on the
coax is very low. The losses in both cases will be lower if you use better coax - using RG-213 reduces the coax
losses to 0.5 dB and 0.1db, respectively. Most hams are happy with those numbers (and many will tolerate
higher losses in exchange for convenience.)
With the same antenna on 15m the situation changes: the SWR is 65 : 1 on the coax running from the rig
to the matching section and the losses in 20' of RG-58 will be about 6.2dB, so 3/4 of your power is lost in
the coax. The feedline loss with the remote tuner is only 0.45 dB. Again, using RG-213 cuts the losses
nearly in half.
(Note that these calculations ignore the losses in the twinlead matching section or in the tuner.)
On some bands where the SWR isn't too bad, the difference in losses between the remote tuner and one
in the shack isn't much. On other bands where the SWR at the antenna is high it can make a big difference,
even in 20' of coax. Trying to match the same antenna on 80m gives you about 92% of your power lost
in the coax with a tuner in the shack (though the losses in a remote tuner should also be considered.) On
the other hand, if you extend the twinlead all the way to the shack instead of using coax, the losses
will be lower - though still significant on 80m.
VK1OD has an excellent analysis of losses in the full-sized G5RV that will apply to your antenna if you double
the frequency scale:http://vk1od.net/antenna/G5RV/optimising.htm
How important is 1dB or 3dB of loss? Half your power (3dB) sure sounds like a lot! But I usually operate at
5 watts output or so and make lots of contacts: that's equivalent to 13dB of loss in your coax to have
the same signal strength. (Not that it is always easy to make contacts, of course.) So it is posisble to
get out even with 10dB or more of coax loss, but your signal will be weaker. If you're trying to break a
pile-up on a rare DX station, that's not a good idea. If you are happy having casual contacts with
relatively strong stations, it may be good enough for you. That's a decision you have to make for