Jim,
Several things are going on here.
First, Antennas are supposed to be resonant and matched to your feedline.
Second and almost always overlooked because wavelengths at HF are so long (I
see massive trouble at VHF / UHF). It is very easy to have your feedline
become an odd multiple of a quarter wavelength.
The old trick with tribanders when you saw high SWR on one band was cutting
a little line off your coax, as the INDICATED SWR would be almost nothing
along the coax somewhere. Even Collins mentioned this trick in the 30L-1
manual. HAHAHA
Of course this would change between different bands. The idea was finding
"Acceptable SWR" somewhere for all the bands. When you have a mismatch
situation, changing the length of your line "Tricks" the reflected power
bridges.
THIS IS ALL GIMICS and isn't addressing your problem.
It sounds like you have a fairly high SWR on your system. First I would
improve the situation at the antenna. That is the correct fix.
When your amplifier is standby the path of bypassing the amp through the amp
will almost always change the SWR indicated by your exciter as the amp is an
impedance bump. Only the "best" of the amp manufacturers were concerned
with this path.
Yes the bridge in your amp might be off. But I'd fist start AT THE ANTENNA.
If your antenna is 50 ohms, the best length of coax is 1/2 wave long
electrically. If you hit an odd multiple of a 1/4 wavelength you get
considerable transformation of the impedance. (Phasing lines are built this
way) This gets way out of hand when you have a mismatch to start with.
Properly tuned antenna, line 1/2 wavelength and even multiples of a 1/2
wavelength long electrically. Never an odd multiple of a 1/4 wavelength.
The end goal is to be able to SELL YOUR SHACK MOUNTED TUNERS ON EBAY.
The only one I use is at the bottom of my 75 foot vertical to tweak it from
one end of 80 and 160 to the other.
Good luck.
BOB DD
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Jim Brown
Sent: Sunday, September 24, 2006 4:03 PM
To: amps@contesting.com
Subject: [Amps] Titan 425 Reflected Power
I'm encountering a strange problem with my Titan. If I follow
standard practice and achieve a 1:1 match with the exciter, then
take the amp out of standby, the amp typically shows mismatch on
the order of 1.5 or 2:1.
If I then re-tune the antenna for minimum reflected power on the
Titan, the VSWR at the tuner, and also at a Bird inserted right
after the Titan, shows a mismatch of 1.5 - 2:1. The mismatch seems
to be greater at higher frequencies.
The bridge in the tuner and the Bird are consistent -- that is,
when the bridge in the tuner shows a match, so does the Bird, 12
inches of RG-8 from the Titan's output connector.
I have three different tuners -- a Ten Tec 229B, a Ten Tec 238A,
and Drake MN2000. It happens with all of them. I've traded cables,
made them as short as possible, changed the switch that switches
between the tuners (from a B&W to an Alpha Delta). The exciter is
going through the Titan to get to the antenna for tuneup. I've
traded Birds. I've eliminated the Bird. All of the cables are RG-8,
all are short.
So you say, go with the bridges. BUT: Grid current in the Titan
tends to be much lower when the Titan bridge indicates zero
reflected power, and rises rapidly when I don't. And grid current
is the big NO-NO in 3CX800A7's, the output tubes in the Titan. I am
careful to keep grid current down to an indicated 30 mA for the
pair (120 mA is absolute max). I can't get close rated power
without excessive grid current unless I tune the antenna for zero
reflected indicated on the Titan.
The SWR indicator in the exciter (TS850) also agrees with the Bird
and the bridge in the tuner -- IF I bypass the amp's RF circuitry
and connect the radio direct to the tuner (or through the
switches). This points to the RF path within the Titan between the
SO-239 output connector and the final output stage. Has anyone
encountered this before? Any fixes?
73,
Jim Brown K9YC
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