It seems that I stirred up a hornets nest with comments about VSWR the
other day. After reading some of the excellent responses and rereading my
own message I see where I wasnt very clear to start with.
Here goes again and I would appreciate any feedback pro or con.
I certainly also agree that at reasonance a 1:1 VSWR is to be expected as
long as the load, the transmission line and the instrument are set at or
designed for the same identical impedence. But, however, a 35 Ohm
antenna, a 50 Ohm line and a 50 Ohm instrument will not read 1:1 at
reasonance. Neither will a 50 Ohm load, a 75 Ohm line and a 50 Ohm
instrument. I am talking about real transmission lines of at least 1/4
wave, not a short jumper. Have I got it right so far?
Now go back to K4AAA's original message. He started off by stating that
the lowest VSWR at 1830 was 1.7:1. Maybe I made a wrong assumption here
but did it mean that that was the lowest VSWR point on 160M? Only Bill
can answer that. Does that 1.7:1 indicate a pure resistance of some value
or are reactances involved? Next, he used a shunt capacitor to obtain a
1.1:1 way up at 1905. Was this at reasonance? I do not think so. Adding
5' of wire to the antenna only brought the 1.1:1 point down 25KHz.
Something is wrong here as only about 1.7' should have been necessary
for 25KHz. Next Bill added a lumped inductance to the base of the
antenna to bring the 1.1:1 point down to 1830....was this resonance?
Again, I do not believe so based upon his following statement that the
amplifier performed a lot different into the antenna as compared to a
dummy load. So now Bill has an almost perfect VSWR but what does it
really mean? I think it is time to go back to the original configuration
with a noise bridge and start from there.
Finally, please note that Bills Ameritron AL-1500 would only load to
1000-1200W with either the original 1.7 or the new 1.1 VSWR. Either that
amp has a severely restricted matching range as a part of its design (Not
specified in the manual) or something out at the antenna is seriously
amiss and the VSWR meter is not telling us what. Remember, that at 100W
of drive the AL-1500 will easily exceed 2KW out when fully loaded but
here it is limping at half of that.
One other comment is that by simply shifting the matching range of the
amp a slight bit will not make it a one antenna amp. With the proper
selection of components it should work quite well into say a 50 Ohm Inv V
and a 35 Ohm vertical. Shifting the matching range to cover a 2:1 VSWR (
I thought most amps could handle this) should not be a big deal. In my
own installation I simply felt it was easier to bugger the amp rather
than trying to Mickey Mouse and weather proof a matching network dangling
10+ feet in the air from a elevated vertical.....just one mans solution
to one problem.
That will be it for tonight gang, lead me down the righteous path!
Tomorrow I will message the results of an experiment this afternoon with
5 different VSWR meters. I am totally confused now!
73.....Carl KM1H
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