[amsat-bb] Re: 1/2 wave bypass for 432 amp?
domenico.i8cvs at tin.it
Sun Dec 9 13:02:18 PST 2007
----- Original Message -----
From: "Bob Stewart" <bob at evoria.net>
To: "Amsat-BB" <amsat-bb at amsat.org>
Sent: Sunday, December 09, 2007 3:56 AM
Subject: [amsat-bb] 1/2 wave bypass for 432 amp?
> My 432 brick has about a 5 DB loss on the receive
> side. I think it's the relay, but getting to that thing
> would be a nightmare and I'm not even sure that
> they're still available. I ran across K3PGP's 1/2
> wave filter, and I see that it is an adaptation of the
> pair of 1/4 wave lines in the old Heath HA-201 amp
> that they used for T/R switching.
> K3PGP's filter:
> HA-201 circuit at bottom of djvu file:
> So to my question: would I likely encounter any
> problems using a pair of 1/4 wave coax lines with
> the diodes in the center to bypass the amp on
> receive? Has anyone done this sort of thing? I
> was thinking of putting it inside the case. Domenico,
> perhaps you? I see your call on K3PGP's page. :)
> Bob - AE6RV
Hi Bob, AE6RV
The circuit that you describes using a pair of 1/4 wave coax lines
with two diodes in antiparallel in the center to bypass an attenuator
on TX/RX was designed for 2 meters by LA8AK and it was described
in OSCAR NEWS No.69 of AMSAT-UK
The same basic circuit is used in to a 10 GHz transverter version 1.0
designed by DB6NT to drive and receive a 2 meter IF with a 2 meters
The principle of working is that if a 1/4 electrical wave coax line
is short circuited at one end (A) it becomes a very high impedance
theoretically infinite at the other end (B)
If you connect the end (B) in parallel to the input of your 432 brick
as soon the amplifier is excited a part of RF will arrive and will be
detected by the diodes connected at the end (A) which becomes a short
so that the impedance at the other end (B) will be very high and the
RF current flowing in to the 1/4 wave line will be only that is necessary
to keep the diodes conducting just above their conduction threshold
and in this case the exciter can only send power in to the input of
The same is through if you connect another 1/4 electrical wave coax
line with one end (C) in parallel to the output of your brick and you
connect the other end in parallel to the same diodes at the point (A)
When power appears at the output of your brick both diodes will be
already in conduction in (A) so that the 1/4 wave line connected to the
output of the brick will show a very high impedance at it's end (C)
and so the brick can only send the power to the antenna.
On the receiving side only very low power is sent to the diodes and
since they are under the threshold voltage not conducting the received
signal is sent to your tranceiver with small attenuation due to a
1/4 + 1/4 = 1/2 wave coax line bypassing the amplifier.
Bypassing the amplifier with a 1/2 wave line directly connected to the
input and output represents a problem because the antenna is looking as
well in to the output of the brick while the tranceiver on receiving is
looking as well in to the input of the brick and probably the old Heath
HA-201 do the same job.
Try to duplicate the circuit of the HA-201 and see what happens but to
separate the input and output of the brick from the 1/2 wave line a couple
of diodes 2 or 4 or 6 diodes in antiparallel depending on the power flowing
should be connected in series to the input and to the output of the brick.
Since the threshold voltage of a diode is in the order of 0.6 volt and
the received voltage is much much lower then no received power is lost
in to the input/output of the brick but it represents a problem from
the point of view of the power to be transmitted and impedance matching
through the diodes.
The 1/4 wave switching circuit seems to be suitable only when a small
power is to be swiched as for example to excite a transverter or linear
from a tranceiver through an attenuator and bypass the attenuator on
receiving without using two coax relays.
For all the above mentioned reason i suggest you to try with the same
circuit of the old Heath HA-201 and in case of no success try to replace
the coax relay with a new one in to your brick and forget the 1/4 wave
project to bypass a power amplifier.
For some PC problem I cannot see the HA201.djvu file but I believe
that the principle of working of the HA-201 T/R switching is the same.
In the original circuit described by LA8AK and published in OSCAR
NEWS No.69 diodes type BA182 (8 in total) where used to switch an
Two diodes in antiparallel are at the center of the two 1/4 wave lines while
4 diodes in antiparallel are in front of the attenuator and 2 diodes in
antiparallel are at the output of the attenuator but instead to use BA182
you can use the same type of diodes as described in the K3PGP line.
To exactly cut 1/4 electrical wave of coax for 70 cm is difficult so that
to introduce a minor error it is best to cut an odd numbar of 1/4 electrical
wave of coax as a 3/4 electrical wave.
Since no power flow in to the 1/4 wave lines it is best to use small
diameter coax cable like RG-188 and solder it directly to the input and
output connectors inside the brick.
Remember that a 1/4 wave coax line shorted at one end show a high
impedance at the other end only at the frequency for wich it is a
1/4 electrical wave so that if it is cut wrong the isolation between
the output and input of your brick is low and damage to your exciter
Inevitably the above lines adds reactances to the circuit so that to retune
the brick for maximum output power is probably necessary.
In a separate email I will send to you the schematic diagram of the
"Transverter Drive Switching by LA8AK" for 2 meters described in
OSCAR NEWS No.69 page 18
I will be very happy to receive from you a drawing and description
of the HA-201 circuit
Best 73" de
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