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G8MNY > TECH 31.03.12 01:02l 149 Lines 5862 Bytes #999 (0) @ WW
BID : 28650_GB7CIP
Read: GUEST
Subj: True VHF Impedance with MFJ259B
Path: IZ3LSV<ED1ZAC<GB7CIP
Sent: 120330/2247Z @:GB7CIP.#32.GBR.EU #:28650 [Caterham Surrey GBR] $:28650_GB
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Mar 12)
(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 & that 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 I placed on the end of the
test lead, gave 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 mask the real impedance.
Ü===============================Z
ÚÄÄÄÁÄÄÄÄ¿ Electoral 1/2 Wave @ F
³R=Z X=0³ 50ê Test lead
³Freq=F ³
ÀÄÄÄÄÄÄÄÄÙ
To nake this I would normally use the quoted velocity factor of the actual coax I
was using. But with this analyser it is easy to cut to length, just use half
the wanted frequency, that gives a short circuit..
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 slighty lower frequency so there is something to cut
off as you get on frequency).
TESTING PRINCIPLE
Now with the lead is made, I was able to confirm the accuracy of my method &
testgear..
Ü===============================?
ÚÄÄÄÁÄÄÄÄ¿ Electrical 1/2 Wave @ F
³Z=? X=0³ 50ê Test lead
³Freq=F ³
ÀÄÄÄÄÄÄÄÄÙ
Any good RF Resistors & combinations (?), I put on the test lead, the Analyser
saw them OK in the range to 0-650ê. And of course a good dummy load read 50ê.
Testing the actual experimental aerial I needed the coax with fly leads for the
screw terminals, but later I terminated the end in a PL259.
AERIAL TESTING & TUNING
Ü===============================[27ê
ÚÄÄÄÁÄÄÄÄ¿ Electrical 1/2 Wave @ F 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 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
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 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! This was convenient to use a PL259 terminated
Aerial Discharge Unit on the boom, The alernative was to use the bigger
waterproof 'N' connectors & a T joiner near the aerial.
The PL259 was sealed from the Wx with water repelant Vaseline inside & then the
outside cleaned & heat glued on 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 shink
was apllied & painted up. This was an alternative to Self Amalgum tape, that
can't be used with water repelant grease.
AERIAL BALUN
The 2 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 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 unbalanced, there
would be a significant difference between the readings.
PROBLEM IN USE
One problem I found using it was due to external high fiends strengths being
picked up by the aerial. In this case the low osc power sent to the aerial was
lower the the RF power being picked up from broadcast masts on the same hill.
Result a misleading high SWR!
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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