Message 28969 in TECH.WW
From: G8MNY
Date: Sat, 29 Aug 26 07:24:00 Z
Newsgroups: TECH.WW
Subject: A Homebrew UHF SWR Bridge
Message-ID: <3606_GB7CIP>
Path: N2MH4|GB7BED|VA2RIZ|VK2RZ|GB7CIP
R:260829/0726Z 36282@N2MH4.#SENC.NC.USA.NOAM LinBPQ6.0.24
R:260829/0725Z 16705@GB7BED.#21.GBR.EU LinBPQ6.0.25
R:260829/0725Z 6718@VA2RIZ.#LAN.QC.CAN.NOAM LinBPQ6.0.25
R:260829/0724Z 21517@VK2RZ.#SYD.NSW.AUS.OC LinBPQ6.0.25
R:260829/0724Z @:GB7CIP.#32.GBR.EURO #:3606 [Caterham Surrey GBR] $:3606_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated May 09)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
For work over 300MHz the standard SWR bridge pickup are too large. The clue to
going higher in frequency is mainly that of size.
Here is a homemade design I use inside kit at 23cms 1st published 5/93 in UK's
BATC CQTV mag No 162 page 25. Although this one was only used @ 24cms the small
size enables it to be used at higher frequencies.
INTERNAL SWR BRIDGE FOR 23cms
External SWR bridges and power meters are ways; fiddly to connect up, lossy,
and expensive pieces of test gear, that you always need connected when they're
not. This bridge design has been used inside "Brick PAs" & "1 Watt exciters".
SMALL
The only difference in a 23cm SWR bridge to a VHF one, is the reduced scale.
So if the miniature bridge pickups are small enough, they can be placed over,
only a few millimetres of 50 ohm track, which can be found on most PA layouts.
As both length of pickup line and frequency determines sensitivity, the small
size is not a problem for picking off enough power to drive the meter OK as RF
just leaks of the PCB!
COMPONENTS
All the components are mounted with minimum lead lengths. There are better
diodes than 1N4148, but these do give surprisingly good performance at these
frequencies.
The forward signal terminating resistor is not critical, but a very small one
should be used in an attempt to get the "100R" required at 1296MHz.
The reflected terminating resistor is too critical to guess at, so a very small
preset trim pot was used. This was a high value plastic 1k, but worked OK. The
RF pickup wires should be about 6mm-10mm long.
TX Screened For
/// AERIAL /// ///
EDGE VIEW OF PCB
For Ref . end on wire pickup near 50R PCB strip line
. . Pot Upper
~~~~-~~-~~~~~~Plain
____50R_______ ~~ copper upper PCB track & ground
P.C.B. Lower
CALIBRATION
1/ Adjust the calibration pot to give no meter reading on a GOOD LOAD.
2/ With reduced output (PA run on 10V) and NO load connected, adjust pickup
distances so that forward & reflected give the same reading.
3/ As 2 but with a FULL SHORT connected, adjust distances for best compromise
in forward and reflected readings.
4/ Recheck 2.
5/ With full power set the sensitivity pot for FSD.
Meter scale
Deflection % 100 80 72 50 33 20 8 0
SWR 1: Inf 9 6 3 2 1.5 1.2 1
Loss dB Inf 4.4 3 1.3 0.5 0.2 .035 0
HOW IT WORKS
The principle to these pick up loop SWR bridges is really the same as the HF
type with RF transfomers etc. except the sensitivity is proportional to
frequency. e.g. 2x frequency = 2x deflection or 4x power! (Bird meters of this
type use lossy dielectric inbetween to flatten the frequency response a bit)
If we look at how a pickup loops sees both the voltage & current components.
/// /// ///
BOTH CURRENT ONLY VOLTAGE ONLY
If we don't have an R the voltage across the loop L is proportional to current
and drives the detector and is current (power) direction sensitive.
If we have just stray capacitance C and no parallel pick up loop, some of the
feeder voltage appears across the R and the detector sees that and as it is
just voltage it is not power direction sensitive.
RF signals are added in series they either add up to double for power going up
or cancel for power going down on transmission line.
Cancelling (SWR 1:1) only occurs if the R value has the same voltage and phase
as the pick up loop for an ideal load. The R can be switched on some SWR
bridges for the values needed for 75R systems.
Note that its is not a "true power meter", as "Voltage + Current" is not the
same as "Voltage x Current", and that can lead to calibration errors on
anything other than "perfect loads".
See also my Tech bul on "Meter Damping & Speed Up".
Why don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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