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G8MNY > TECH 24.08.20 16:18l 158 Lines 6527 Bytes #-1529 (0) @ WW
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Subj: True VHF Impedance with MFJ259B
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From: G8MNY@GB7CIP.#32.GBR.EURO
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By G8MNY (Updated Dec 16)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
I have used this SWR Analyser instrument to verify & tune up a new commercial
VHF Broadcast aerial for a local community radio station. I needed to confirm
it was on resonance, the aerial design Z was as predicted (27ê) from aerial
design (MM-ANA-GAL), & that my Matching & Balun to 50ê unbalanced, was all
working correctly to help assure the predicted polar diagram would be met too.
MAKING A HALF WAVE LENGTH TEST LEAD
At VHF this is the starting point, so that anything placed on the end of the
test lead, gives a true reading, without the need for Smith chart coax
transformations.
Any multiple of an electrical half wave length will do this (1, 1.5, 2 etc.),
but coax losses can mask the real impedance.
Ü====================================Z
ÚÄÄÄÁÄÄÄÄ¿ Electrical 1/2 Wave @ F MHz
³R=Z X=0³ 50ê Test lead
³Freq=F ³
ÀÄÄÄÄÄÄÄÄÙ
To make this lead I would normally use the quoted velocity factor of the actual
coax. But with this analyser it is easy to cut to length, for a half wave, just
use half the wanted frequency, that gives a short circuit reading..
Ü====================================open
ÚÄÄÄÁÄÄÄÄ¿ Electrical 1/4 Wave @ F/2
³Z=0 X=0³ Cut length to give Z=0
³Freq=F/2³
ÀÄÄÄÄÄÄÄÄÙ
Measuring the length also confirms the coax velocity factor & termination
length errors. (Start at a slightly lower frequency so there is something to
cut off as you get on frequency).
TESTING PRINCIPLE
Now with this test lead made, I was able to confirm the accuracy of my method &
testgear..
Ü====================================?
ÚÄÄÄÁÄÄÄÄ¿ Electrical 1/2 Wave @ F MHz
³Z=? X=0³ 50ê Test lead
³Freq= F ³
ÀÄÄÄÄÄÄÄÄÙ
Any good RF Resistors & combinations (?) I put on the test lead, the Analyser
accurately saw it, in the range to 0-650ê, & of course a good dummy load reads
50ê.
Testing the actual experimental aerial, I needed the coax with fly leads for
the screw terminals, but later I terminated it in a PL259.
AERIAL TESTING & TUNING
Ü====================================[27ê
ÚÄÄÄÁÄÄÄÄ¿ Electrical 1/2 Wave @ F MHz Test
³Z=27 X=0³ 50ê Test lead Aerial
³Freq= F ³
ÀÄÄÄÄÄÄÄÄÙ
Using the test lead for measurement gave pretty close to the theoretical Z for
the aerial, but I had made it a bit bigger on purpose, so the reactance X was 0
at a lower frequency. I was then able to cut the aerial shorter until the
resonance (X=0) was exactly on frequency.
Some of the more accurate work was done outside, to eliminate the building
affecting things, e.g. any approach to either end of the elements saw quite a
change!
AERIAL MATCHING
As this was to be a permanent aerial, & I did not want exposed gamma matching
joints etc, so I used 1/4 wave of 37.5ê coax, which is actually 2x 75ê in
parallel. Tuning up the length of this was done in the same way as the test
coax except as 1/4 wave was wanted F was used...
Ú===========================open
ÚÄÄÄÁÄÄÄÄ¿ 1/4 Wave 75ê @ F MHz
³Z=0 X=0³ Cut length to give Z=0
³Freq= F ³
ÀÄÄÄÄÄÄÄÄÙ
Then using 25ê load the matching was tested..,
===================
Ü===================================Ü[===================]25ê
ÚÄÄÄÁÄÄÄÄ¿ Electrical 1/2 Wave @ F MHz 1/4 wave 2x 75ê
³Z=50 X=0³ 50ê Test lead
³Freq=F ³
ÀÄÄÄÄÄÄÄÄÙ
The 2 matching coaxes were put into a PL259 plug, well you can't get 2 coaxes
into one plug other than a PL259! It was convenient to use a PL259 to
terminated onto an "Aerial Discharge Unit" bolted on the boom, The alternative
was to use the bigger waterproof 'N' connectors, & a T joiner near the aerial.
The PL259 was sealed from the Wx with water repellent Hot Vaseline inside, &
then the outside cleaned, & heat glued in the cable gap, & a heat shrink
sleeves put over the joint.
1st Glue 2nd Shrink Glue
Glue Shrink Glue \ _____________ /
\_______/ _____ \Ü______ ___Ü/
ÜÜÜÜÛÜÜÜ--ß-Ù-Ü ÜÜÜÜÛÜÜÜ--ß-Ù-Ü
glue gap>ÜÜÜÜÜÜÜÜ______Ûßßß ÜÜÜÜÜÜÜÜ______Ûßßß
-ß-----ß ÀÄÄÄÄÄ Û-----ß ÀÄÄÄÜ
Greased up plug -------------
Once screwed up tight on the Aerial Discharge Unit, a 2nd glued on heat shrink
was applied over the lot & painted up. This was an alternative to Self Amalgum
tape, that can't be used with water repellent grease.
AERIAL BALUN
The 2x 75ê matching coaxes were coiled up to make a choke Balun. The whole lot
was cable tied up close to the connection box to make the coils stay in place &
painted to stop movement & UV damage etc.
The plug was connected to an Aerial Discharge Unit bolted to the boom. This
ensured there would be no residual RF on the coax feed outer.
|
| test aerial
|
Choke | \Touch
Ü===================================Ücc(:) either
ÚÄÄÄÁÄÄÄÄ¿ 1/2 Wave @ F MHz ßßßßßßßßßßßß| /side
³Z=50 X=0³ 50ê Test lead boom |
³Freq=F ³ |
ÀÄÄÄÄÄÄÄÄÙ |
The effectiveness of the balun could be seen with the Analyser by touching the
driven element either side of the connection box (:), it give the same SWR
readings. If the choke balun was not working, or the aerial was slightly
unbalanced, there would be a significant difference between the readings.
PROBLEM IN USE
One problem I found using the MFJ Analyser on a hill top, was due to external
high field strengths being picked up by the aerial. In this case the low osc
power sent to the aerial was lower than the RF power being picked up from
nearby broadcast masts on the same hill. This resulted in as a misleading high
SWR & time wasted faulting "Red Herrings".
See my Tech Buls "PL259 Losses", & "Coiled Coax Choke Balun for VHF"
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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