Posted by
Steve Banks on
Apr 22, 2005; 12:00am
URL: http://elecraft.85.s1.nabble.com/no-subject-tp377455.html
Hi Everyone-
I'm looking for any thoughts or suggestions from anyone about some recent
VSWR tests I've made on two antennas.
GAP TITAN BACKGROUND: I've had a GAP Titan vertical antenna running with my
K2 (S/N 1599) for several years. The K2 and the GAP have served me very
well (QRP WAS, 28 countries, etc.). The GAP is fed with about 80 ft of RG8U
type coax (Belden 8214). Its VSWR measurements look completely normal to me
when measured from here in my shack using an MFJ 259B meter. For example,
typical VSWRs on 20m range from about 1.7 to 2.1 measured directly at the
feedline PL259 connector. Most of the other seven GAP bands display similar
"expected" VSWR ranges.
Here in my shack the GAP feedline connector is attached to the center
(common) port on a manual (not electronic) Delta-4 coaxial switch. The K2
and a couple of other rigs (Drake and Heathkit) are connected to the
switchable Delta-4 ports. That arrangement enables simple rig switches to
the GAP antenna without having to move the feedline from rig-to-rig.
Measurements of feedline VSWR made THROUGH the Delta-4 switch to the GAP
antenna with a 259B meter produce very similar results to those made
directly at the feedline connector (i.e. with the switch bypassed).
Conclusion: the Delta-4 switch seems completely transparent to the
rig/feedline system. So far so good.
HEXBEAM RECENT ADDITIONS: I recently put up a five-band HexBeam (2
wire-type elements for each of the five bands, 20m-10m). It seems to
perform very well. The K2's KAT2 tuner likes it, as do the two other rigs
and tuners I have (Drake and Heathkit). In the process of finishing the new
Hex antenna, and "shaking down" its performance after installing it, I
discovered one defective feedline PL259 connector which I replaced
(naturally, outdoors on a windy day).
Following that, I measured the HexBeam's VSWR across its five bands from the
shack feedline connector with the 259B; all was well with it. VSWRs
measured between 2.2 and 1.5 with no strangeness or instability seen on any
of its five bands. Values on 20m, for example, ranged from 2.2 down to 1.7,
depending on frequency. Similar measurements show up on the Hex's other
four bands. (The top of 10m is somewhat higher, but that's not my issue.)
MY ISSUE: Even though the HexBeam's 20m VSWR measurements appear quite
normal made directly at its shack feedline connector using the 259B meter,
they are VERY DIFFERENT when measured through a Delta-4 switch. The GAP
antenna's VSWR's are very normal-looking when made through its Delta-4
switch, but the HexBeam and 259B meter are NOT HAPPY with a Delta-4 switch
in place. (The HexBeam's Delta-4 switch is actually a separate switch, not
the same one the GAP is connected to.)
For example, instead of seeing a 20m range from 2.2 down to 1.7 on 20m
WITHOUT the Delta-4 switch in place, I see values from about 4.5 or 5.0
across the whole band WITH IT IN PLACE. The 259B meter also appears quite
"skittish" making measurements through the switch. It seems unable to
decide on R,X and SWR values that remain reasonably stable.
Something about the Delta-4 switch seems to have upset the HexBeam's VSWR
applecart when being tested with the 259B. I see no such strangeness when
measuring through the switch to a dummy load or to the GAP antenna when
connected to the same switch. The only unexpected results seen through the
switch are those made to the HexBeam.
Just for jollies I decided yesterday to ignore the apparent problem, and
just let my K2 and other rigs run on the HexBeam, each with its own tuner.
All were happy, with the HexBeam's directivity quite obvious on all of them.
I've disassembled both of my Delta-4 switches, cleaned their contacts,
verified that all of my cable connections to the switches are secure and
correct, and that no other dumb-thumbing factors have been introduced by me.
I've done all the usual loosening-wiggling-tugging efforts to see if I can
find a cable connector that might be in trouble, but nothing has shown up
yet.
MY QUESTION: Has anyone else ever used Delta-4 switches and seen similar
unstable/unexpected VSWR results, either with a HexBeam or some other
antenna connected to it? It seems obvious that the switch, only when
connected between the HexBeam and a rig, introduces some sort of VSWR
increase, but I've run out of ideas for what to try next. Yesterday my
granddaughter said, "Just quit measuring and start QSOing!" Ah, the wisdom
of a 12-year-old, huh?
Thanks and 73 to all for the bandwidth and for any thoughts.
Steve Banks
K0PQ
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