[CQ-Contest] What RF voltage OK at Rx in SO2R?
Marijan Miletic, S56A
artinian at siol.net
Wed Sep 26 17:20:14 EDT 2001
Most RX would suffer at 1 V close RF input, long before diodes would
conduct!
Two pairs of back-to-back diodes are standard protection on VHF RX.
Good front-end filters are the only SO2R-enablers.
73 de Mario, S56A, N1YU
P.S. I can't even find a word "tad" in my dictionary!
----- Original Message -----
From: Mike Gilmer - N2MG
To: cq-contest at contesting.com
Sent: Wednesday, September 26, 2001 12:45 PM
Subject: Re: [CQ-Contest] What RF voltage OK at Rx in SO2R?
Even if you don't care about the "re-radiating" of crud, if you are trying
to actually listen with the radio that has been protected this way, AND the
diodes are conducting - from the TX sigs from your other radio(s) - you
might be a tad disappointed.
The circuit is a failsafe, but not an SO2R-enabler.
Mike N2MG
On Tue, 25 September 2001, k8cc wrote:
>
>
> While the circuit G4BUO described will most certainly protect the
receiver
> front end, it will also likely have the nasty side effect of generating
> spurious signals all over the spectrum. When the diodes conduct from
the
> RF coming in the antenna, the spurious signals are re-radiated out the
> antenna. If you live out in the country (as I recall Dave does from my
> visit to his QTH in 1990) perhaps this is no big deal. In a suburban
area,
> I would think that every time you transmit you'd wreck every television,
or
> amateur, over a wide area.
>
> Back when I first got going at this QTH, I had a 2L 40M quad switchable
> NE/SW using a relay on the tower between the loops. When the quad was
in
> the default (relay relaxed) position, there was a S9+20dB harmonic on
> 14.200 (not even the band of the quad!) which absolutely disappeared
when
> the quad was switched SW (relay actuated). The problem turned out to be
a
> diode across the relay coil to protect against "flyback" transients when
> the relay was de-energized. The 300' of twisted pair control wire from
the
> shack to the relay picked up enough RF from a 10 KW BC station *three
miles
> away* on 1420 KHz to generate a *tenth* harmonic on 20 SSB. It also
caused
> the only two TVI complaints I've ever had from this location in fifteen
years.
>
> Don't use diodes to protect front ends unless you're also going to have
> filtering to stop the spurious signals generated.
>
> Dave/K8CC
>
> At 03:57 AM 9/25/01 -0400, Dave Lawley wrote:
>
> >Here's an RX protector circuit I made up a couple of years
> >ago, following a posting here on the reflector by Tom N4KG.
> >Brought it with me to WRTC2000 and we used it on the second
> >rig, though the voltage levels didn't really warrant it.
> >
> >Here at home I managed to squeeze a 260ft beverage
> >diagonally across my garden, but it was only about six feet from
> >my 40m 4-square at the nearest point. The bulb lights brightly
> >when I'm transmitting on 40m but the front-end of the FT1000MP
> >has held out fine.
> >
> >Here's the circuit. The bulb is a 60mA grain of wheat type and
> >the whole thing is mounted in a metal box with a small hole so I
> >can see the bulb when it lights.
> >
> >Dave G4BUO
> >
> >
> > antenna --- bulb --- 0.1uF --- 33ohm 2W ------------- rx
> > ! !
> > ! !
> > ! ! back-to-back
> > ! ! 1N914 diodes,
> > ! ! 4 in each string
> > ! !
> > ground --------------------------------------------- gnd
> >
> >
> >--
--
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>From Silver Ward" <hwardsil1 at mindspring.com Wed Sep 26 16:37:04 2001
From: Silver Ward" <hwardsil1 at mindspring.com (Silver Ward)
Date: Wed, 26 Sep 2001 16:37:04 +0100
Subject: [CQ-Contest] Diode-Generated Crud
References: <4.2.0.58.20010925230422.00acf4a0 at pop.mw.mediaone.net>
Message-ID: <008501c146a1$19ac8460$842379a5 at ward>
Many rotor control boxes and other antenna-related accessories also have
unbypassed rectifiers in them that can generate harmonics and mixing
products.
As K8CC points out, the products can also cause TVI for nearby neighbors
because they are in-band signals to the TV and can NOT be filtered out with
a high-pass filter. A spectrum analyzer sweep of your output signal will
show it to be clean, but the products can definitely be caused by your
transmit signal . Unbypassed diodes in the neighbor's equipment can also
cause the products - disconnect antenna or other cables to see if a device
(even one that is off) is causing the problem.
Most single-op stations do not have to be concerned (except about TVI)
because the products are only generated when the transmitter is on (and the
receiver is off). However, if you are considering SO2R or a multi-station
operation, this can be a real problem. Not only will a transmitted signal
generate harmonics in the diodes, but more than one transmitted signal will
mix and generate all sorts of intermod that can't be cured with stubs and tx
output filtering.
In my case, the culprit was a T2X control box generating crud on 160 through
40-meters. I live near several AM BC stations and there was enough RF on
the control cable to generate substantial intermod products without my
transmitter being on. I only discovered this when I noticed that the
products were greatly reduced when I rotated the antennas - which either
cuts off or turns on the rectifiers enough to get them out of their
non-linear region. The cure was to bypass each line in the rotor control
cable to the chassis with a .01uF/50V (anything from .001-.1uF will do)
capacitor and then to ground the chassis. I did this on all three control
boxes in the shack and have also made sure that all relay kickback diodes or
current steering diodes are bypassed in all antenna or rotor accessories.
Band-pass filtering of a receiver's front end may not cure in-band mixing
products from protection diodes. Strong local signals on the same band will
mix in the diodes and generate odd-order intermod products. (This isn't a
problem except for multi-station environments or for those with nearby
strong stations.) Grain-of-wheat light bulbs are usually sufficient and are
sufficiently linear to avoid generating nasty mixing products.
73, Ward N0AX
----- Original Message -----
From: k8cc <k8cc at mediaone.net>
> While the circuit G4BUO described will most certainly protect the receiver
> front end, it will also likely have the nasty side effect of generating
> spurious signals all over the spectrum. When the diodes conduct from the
> RF coming in the antenna, the spurious signals are re-radiated out the
> antenna.
>
> Back when I first got going at this QTH, I had a 2L 40M quad switchable
> NE/SW using a relay on the tower between the loops. When the quad was in
> the default (relay relaxed) position, there was a S9+20dB harmonic on
> 14.200 (not even the band of the quad!) which absolutely disappeared when
> the quad was switched SW (relay actuated). The problem turned out to be a
> diode across the relay coil to protect against "flyback" transients when
> the relay was de-energized
--
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