There have been a number of discussions about the accuracy of the Orion 1
SWR meter. My feeling has been that it is not a precision instrument, but
recently I have been puzzled about a situation in which I am seeing
something other beyond an approximate but fairly close reading. I use an
Alpha 86 and an Alpha 89 amp at various times. I do not usually look at the
Orion SWR meter to see how the Alpha looks to it, but on occasion I have
done so and concluded the readings were between 1:1 and 2:1. However I have
noticed a few times when the indicated SWR when driving the Alpha was 2.2:1,
2.4:1 or as high as 3:1. The 86 was in the hospital recently for a blower
change and I mentioned that the input SWR seemed a little higher than I
expected. The report back was that the SWR was usually very close to 1:1
and never above 1.5:1. When I had seen the higher readings there was no sign
of the Orion having a problem driving either amp, folding back the power,
etc, which I felt was odd. I don't like to see a transceiver stressed with
SWR more than about 2:1 as my feeling is that is asking for trouble. A
number of times when I had seen a high indicated SWR I had switched in the
internal tuner and found it quickly tuned-not the case when faced with a
really strange load. Again I felt this was a little odd.
Knowing that the SWR seen by the exciter can be affected to a certain extent
by how the amp is tuned and loaded, and that sometimes there could be an
issue with the length of the cable between the rig and amp (several folks
have told me that ETO used to suggest a magic length somewhere around 5
feet), I began to play around a little when the amp came back. I had about
a two foot cable between the amp and the rig, and I decided for the heck of
it to connect the drive cable to a Coaxial Dynamics power meter with a 100W
30MHz slug in it and to use a 3' cable on the other side to connect to the
amp. I found that though the forward power indicated by the meter is what I
would expect for the amp power out, there is hardly a discernable wiggle of
the needle when measuring reverse power. When the Orion tells me it sees
1.0:1 there is no deflection, and when it tells me it sees 2.4: 1 there is
still no sign of reflected power. If I keep the amp in the standby position
so that I am looking at good but not perfect antennas I do see the expected
reflected power on the Coaxial Dynamics. With the amp in standby mode, I
also see an approximately correct SWR on the limited accuracy Orion meter
when looking at the imperfect antenna.
I have seen a number of posts regarding the Orion seeing higher than
expected SWR when driving an amp and questions about use of the internal
tuner in such cases. But I am wondering now if these cases might be like
mine, with the meter apparently more insane than usual when the Orion is
driving an amp. I can't come up with a good explanation of why the Orion
meter would show much worse accuracy when driving an amp. I have wondered
if somehow RF could be getting into the Orion and affecting the reading, but
I really haven't come up with a decent explanation. By the way, my antennas
are distant and I use common mode chokes on all of them; I also have ferrite
on the rig to amp drive cable, given the Orion's tendency to do strange
things when exposed to noise, RF, etc-at least with earlier firmware (I have
stayed with 1.372). Does anyone on the reflector have insight into what
could be causing this?
Bob W2WG
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|