I used a return loss bridge and measured a TEE fitting with a good 50
ohm load on one port the length of cable on the other side. The antenna
analyzer will read a perfect 50 ohm match when the cable is 1/2
wavelength long. If the cable is an open quarter wave, your analyzer
will show high VSWR or low return loss. keep in mind that you will see
many bumps as you increase frequency and you get multiples of half waves
and quarter waves. You want the lowest frequency VSWR null and that is a
single half wave at that frequency. A half wave repeats the impedance,
so an open half wave in coax will also look like a open at the TEE
fitting and it is across the 50 ohm load so it still looks like a 50 ohm
load. (low VSWR) Every open quarter wave coax reflects back as a short,
and that makes the VSWR very poor.
Dave K1wHS
On 6/9/2018 7:55 PM, terry burge wrote:
Hi guys and gals,
I have got DX Engineering 75 ohm foam coax to make the stubs/feeders for my
Comtek 80 meter 4-Square and am finding some difficulty. Neither my MFJ-259B
nor my Rig Experts AA-170 seem to indicate a 1/2 WL at 7500 KHz or a 1/4 WL at
3750 KHz? Or any frequency up or down from there. Seems to me the last time I
did this a few years ago I didn't have a problem getting a 1/2 WL dip
indicating a resonance location. But now I just don't get any indication of a
resonance.
DX Engineering said they had got some 75 ohm foam coax that instead of 0.84 VF
was more like 0.80 down to 0.76 VF. But checking all the way down to 6000 KHz
on 40 meters or 2500 KHz on 80 meters fails.
My test is to use either antenna analyzer with a PL-259 installed at one end and
an open or a short at the other. I started with 57'7" of coax. Tried both with
the coax in a roll and strung out. What am I doing wrong?
Terry
KI7M
😞
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