| |
G8MNY > TECH 30.04.09 08:26l 75 Lines 3017 Bytes #999 (0) @ WW
BID : 24916_GB7CIP
Read: GUEST
Subj: A Homebrew UHF SWR Bridge
Path: IZ3LSV<IK2XDE<DB0RES<ON0BEL<ZL2BAU<N1UAN<GB7CIP
Sent: 090429/2158Z @:GB7CIP.#32.GBR.EU #:24916 [Caterham] $:24916_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New Jan 09)
(8 Bit ASCII Graphics use code page 437 or 850)
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, &
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 & frequency determins 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 "100ê" 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
ÚÄÄÄÄÄÄÄ Û ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄo \_______
ÚÄÄÄÄÄ)ÄÄRÄÄ¿ Û ÚÄ´>ÃÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄo ³
³ ³ ³ Û ³ ³ Leads Ref 10K <ÄÄÄ¿
³ ³ ³ Û ³ === C POT METER
³ C ³ ³ Û ³ ³ n1 ³ 100uA
ÃÄÄ´ÃÄÁÄ´<ÃÄÙ Û ÀÄPOTÄÄ´ ÃÄÄÄÄÄÄÙ
_³_ n1 Û _³_ _³_
/// AERIAL /// ///
EDGE VIEW OF PCB
For Ref
. . Pot Upper
/ \± Ground
~~~~-~~-~~~~~~Plain
____50ê_______
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) & 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 & reflected readings.
4/ Re-check 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
Why don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
Read previous mail | Read next mail
| |
