Repeater Isolation without a Duplexer
Christopher E. Piggott
My university club has basically decided that getting a 2m duplexer is
not only a pain in the neck, but it's also expensive...we decided that
a better alternative (at least from a $$$ standpoint) will be physical
separation of the Rx/Tx antennas.
According to the book, at 146Mhz, 300' horizontal spacing will yield
60db isolation with a standard 600kc split. According to those specs,
a 40' vertical spacing will also yield 60db isolation.
My two questions are:
(1) What is the relationship between spacing and isolation
if we do BOTH - i.e. 300' horizontal and 40' vertical?
I don't really believe that 60db from the horizontal
plus 60db from the vertical would add up to 120db
total isolation - but I don't really know!
(2) We are planning on using Ringo Ranger II's. I don't
understand the relationship that the 7db gain
the Ringo Ranger has will do to the isolation
figures - i.e. will 7db gain, does that imply
a 7db loss of isolation on the receiver? (if
"gain" is the opposite of "isolation", that is,
will they cancel each other in a linear sort of
fashion?)
Chris, N2JGW
UUCP: ..rutgers!rochester!ritcsh!kodak!pcid!gizzmo!lazlo!cep
--
Christopher Piggott, N2JGW, President
Rochester Institute of Technology Amateur Radio Club, K2GXT
One Lomb Memorial Drive, Rochester, NY 14623 / (716)-475-6973
Bob Witte
I've tried the two antennas instead of a duplexer technique before
and got nothing but grief. It is difficult to maintain the required
isolation and the differing antenna positions tends to compromise
the coverage. (The coverage is usually not balanced on transmit and
receive.) Other people have related similar problems to me. I would
suggest coughing up the money for a good duplexor instead.
Anyone out there in netland have any better luck with the two-antenna
solution?
Bob KB0CY
John Boteler
> My two questions are:
> (1) What is the relationship between spacing and isolation
> if we do BOTH - i.e. 300' horizontal and 40' vertical?
> I don't really believe that 60db from the horizontal
> plus 60db from the vertical would add up to 120db
> total isolation - but I don't really know!
> (2) We are planning on using Ringo Ranger II's. I don't
> understand the relationship that the 7db gain
> the Ringo Ranger has will do to the isolation
> figures
1. It is much easier to achieve vertical isolation, say on a tower,
for your purposes than to achieve horizontal separation.
There are still near-field effects which may cause
duplex noise if you have any rust on the tower, but
they will most likely be less than if you duplex
on one antenna. Put the receive antenna way up high
and mount the transmit antenna a good ways below it.
As soon as the two antennas are no longer aligned
in the same vertical line, their fields will begin
to interact much more and drastically reduce the
effectiveness of vertical separation. If you really
must have horizontal separation, do it right and
get a mile or so between them with a phone line
or RF link-another expense and a big maintanence
headache!
2. A Ringo Ranger?! After you find a real antenna, the gain
figures for it will indicate how much it compresses
the standard 'figure 8' dipole pattern down into
an elliptical pattern; that is where 'gain' comes
from. It is gain referenced to another antenna's
pattern. Since a high gain vertical dipole array
squeezes the pattern in the vertical plane such
that it is much weaker off the horizon, it has the
effect of extending the reach of the pattern out
toward the horizon, much like squeezing a round
balloon until it takes the shape of a bullet.
I find vertical dipoles are the way to go. If they require
repair, the wiring harness is usually the culprit and
is usually accessible externally. If the elements bolt
on to the mast with clamps or similar fasteners, it is
a relatively simple job to tighten them; these are the
two biggest sources of noise on these antennas. Plus
you get a free DC ground with every one!
--
Bote
uunet!cyclops!csense!bote
{mimsy,sundc}!{prometheus,hqda-ai}!media!cyclops!csense!bote
Strawberry Jammer
> I've tried the two antennas instead of a duplexer technique before
>and got nothing but grief.
...
>Anyone out there in netland have any better luck with the two-antenna
>solution?
No in fact of the dozen or so times I have been involved none have worked
well enough to be worth the trouble (I include my full duplex remote base
here too). The only times it DOES work is crossband - 2M to 432 or some such,
or by using sites separated by more than a mile.
The one time the separate sites were used was a cable TV receiving location
and they were adamant about NO transmitters (the link was landline). It
worked, but the coverage was very strange. We ended up using the remote as an
auxiliary receiver with a fairly high powered/low sensitivity setup on the
mountaintop (together with almost giga-watt ERP from TV, FM broadcast etc).
The transmit site was about 200 yards across with almost 50 transmitters
including 5 TV channels, intermod was unbelievable!
In this case the remote receiver worked well for weak/dx signals and the
mountian top receiver worked well for "downtown" fill in coverage. I think
they ended up with the local receiver having priority though. Not a simple
set up and the chief engineer for one of the FM stations was club trustee!
Get a duplexor!
--
*Mike Waters AA4MW/7 wat...@dover.sps.mot.com *
Menu, n.:
A list of dishes which the restaurant has just run out of.
Chris Kelly
repeater running without duplexer using vertical
separation. In fact, the receiver has a ARR GaAs preamp
at the antenna, and there are no desense problems.
The receiving antenna is at the top of a 125' tower,
the transmitting antenna is about 50' up.
Keys to success:
1. Real antennas (ie Celwave Stormchief on the Rx,
Celwave Stationmaster on the Tx.)
2. Good feedline (3/4" Heliax). You can forget isolation
completely if you try to use normal Coaxial cable.
3. Good Tx/Rx isolation all the way to the power supply.
(RF leakage in the shack can be just as big a problem
as coupling on the tower).
4. Good vertical separation (about 70' in this case).
5. Good commercial receiver (GE, in this case) with
helical resonator in teh front end.
6. No other transmitters on the same band at this site.
This is one reason the unprotected preamp could be used.
The preamp (NOT usually a good idea on repeaters) was
added because of the necessity of driving about 200' of
feedline from the receiver.
In most cases, I think you are better off springing
the $500 for a cavity duplexer (on 440 MHz, more on
lower bands). Also remember that Tx/Rx RF isolation MUST
include all wiring in the repeater: Audio, Power,
controller, etc. This (and continuous duty Transmitters)
is what distinguishes a well made repeater from any old
mobile rig.
73, de WD5IBS............chris
John Boteler
> [ thorough discussion of repeater isolation issues deleted ]
> In most cases, I think you are better off springing
> the $500 for a cavity duplexer (on 440 MHz, more on
> lower bands). Also remember that Tx/Rx RF isolation MUST
> include all wiring in the repeater: Audio, Power,
> controller, etc. This (and continuous duty Transmitters)
> is what distinguishes a well made repeater from any old
> mobile rig.
I heartily agree!
Keep in mind, however, that being in the right place at the
right time can reap great rewards in the hunt for commercial
grade repeater equipment.
I have purchased and seen duplexers at hamfests for as little
as $50! These were in working order, but were either on an
unpopular band or split, or needed recabling--not a trivial
operation, but not impossible, either.
There are other deals to be had if you keep a vigilant eye
out. This past Sunday I picked up the essentials of a
Motorola Spectra-TAC voting system for $2.25 ! Yes, the
decimal point is after the first '2', and the guy
even threw in a full-height double sided floppy drive!
What a deal!
Good luck...