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Author Topic: 2M repeater tuned cavitys or 2 antennas  (Read 29519 times)
KJ6HYC
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Posts: 103




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« on: July 11, 2011, 07:48:32 PM »

I am on the technical committee in our radio club. The current repeater isn't owned by the club and the club will be moving it and replacing it. One of the major costs are the tuned cavitys. I have been told by a repeater sales company that if we run 2 antennas that are seperated we don't need the cavitys. What are the pros/cons of the cavitys vs. the dual antennas. What distance are we talking about in antenna seperation (3-4 dB antennas)?

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N2HBX
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Posts: 162




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« Reply #1 on: July 12, 2011, 06:38:41 AM »

The separation distances required for even minimal acceptable isolation at 2 meters makes the two-antenna approach impractical. For example, you would need over 50 feet of vertical separation to obtain about 65 dB of isolation. Horizontal separation would be even further. The two-antenna approach is much more feasible at 440 and above.

You could buy a very nice duplexer for less than it would cost to implement the dual antenna system, plus you wouldn't need all the "real estate" to install it. Maintenance with a duplexer is a non-issue. Once it's tuned and in place, it's basically a "set it and forget it" deal. Our group bought a 4-cavity duplexer for under $1000 that works perfectly.

73,
Larry, N2HBX
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WB2WIK
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« Reply #2 on: July 12, 2011, 08:59:43 AM »

Everything's possible but getting a repeater to function well using split antennas instead of a duplexer is pretty hard and takes experience and experimentation.

It almost always creates an "unbalanced" system, since one antenna will be higher than the other.  To get enough spacing on 2m to make this work, one antenna would normally be about 100 feet higher than the other.  If that's the TX antenna, the repeater will get out a lot farther than it can hear; if it's the RX antenna on top, it will hear better than it gets out.

One way this can work pretty well is "split site," where the separation is quite far (can be a mile or more) and the RX signal is linked back to the TX site via a higher frequency ham band link, or via telephone, or via the internet.  That almost always results in an "unbalanced" system, but at least it works, and you don't run into desensitization and noise issues.

A duplexer's a lot easier and only requires one antenna and one transmission line.  When one considers how much good antennas and coax cost, that savings could be invested in a duplexer to have a better system for not much more money -- in some cases, maybe even less money.
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AD4U
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Posts: 2186




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« Reply #3 on: July 12, 2011, 10:01:25 AM »

Don't skimp on the duplexer either.  If you want the repeater to perform right, this is not the place to use cobbled together cheap hamfest bits and pieces (junk).  

How much isolation you need depends on several factors (transmitter power output, transmitter purity, receiver sensitivity, receiver selectivity, leakage in connecting cables, isolation in the antenna, just to name a few).  

In a typical low power (30 watt) repeater system 95 dB isolation in the duplexer is generally considered minimum to eliminate desense, if everything else is "right on".  Beware that desense can be caused by other factors even if you have adequate duplexer isolation.  

On my two meter repeater I use a Celwave 6 section duplexer with a measured 120 dB isolation on my HP 141T.  The repeater is a GE Mastr II running 100 watts to a Super Station Master antenna with no desense at all.  

We have enjoyed using the repeater for more than 20 years - long after the financial pain of buying the duplexer (and the antenna) was forgotten.

Dick  AD4U
« Last Edit: July 13, 2011, 09:59:45 AM by AD4U » Logged
WB2WIK
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« Reply #4 on: July 12, 2011, 01:57:22 PM »

Don't skimp on the duplexer either.  If you want the repeater to perform right, this is not the place to use cobbled together cheap hamfest junk. 



To make a "good" single-site repeater not only takes a good duplexer, but the right kind of antenna and coax, and quite often a circulator (if it's a shared site with other transmitters) as well.

It helps to have a professional repeater guy in the club to help with the hardware selection and installation.  I've seen clubs pour good money after bad into repeaters because the original selections weren't smart, and it can take a year or more before the system really works properly.
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KJ6HYC
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« Reply #5 on: July 13, 2011, 04:52:45 AM »

Don't skimp on the duplexer either.  If you want the repeater to perform right, this is not the place to use cobbled together cheap hamfest junk. 



To make a "good" single-site repeater not only takes a good duplexer, but the right kind of antenna and coax, and quite often a circulator (if it's a shared site with other transmitters) as well.

It helps to have a professional repeater guy in the club to help with the hardware selection and installation.  I've seen clubs pour good money after bad into repeaters because the original selections weren't smart, and it can take a year or more before the system really works properly.

The problem is we are a small club and no one has any experance w/repeaters. The dual antenna was idea was suggested by a repeater sales person, however we are researching and have not commited any funds yet. I am going to recommend that we should go w/the cavitys.

Thanks for all the great posts.

73

Wayne - KJ6HYC

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AD4U
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« Reply #6 on: July 13, 2011, 10:13:26 AM »

Wayne:  I own four repeaters and I keep two more running for our club.  As others have said, getting a repeater performing properly and making sure it stays that way takes a bit of knowledge, work, and access to some expensive test equipment. 

Based on all the mistakes I have made over the past 40 years I suggest that you and your club try to find a person to help you, who is familiar with the proper design of repeaters.

One more suggestion is to use "retired" commercial equipment.  IMO it will outperform all of the brand new HAM repeaters for a whole lot less money.  All of my repeaters are GE Mastr II's.  The 100 watt continuous duty models can be had for around $200.  It takes a bit of knowledge to get them up and running on the ham bands, but they do not have to be modified - just retuned. 

In addition to a good duplexer you will need the "right" antenna.  If you choose the wrong antenna that does not have adequate decoupling from the feedline, you will probably have desense issues with your repeater that will drive you crazy.

Go to  "repeaterbuilder.com" and read the info presented there.


Dick  AD4U
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W8JI
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« Reply #7 on: July 15, 2011, 02:04:14 PM »

Wayne,

If you side mount both antennas on the same side of the tower you will barely see any differences. All you need to do is put the TX on the lower antenna and bump up power a bit.

If you use something like an old Motorola Micor you will be able to duplex with as little as 50 feet of vertical separation. Othere than a little TX composite noise it is one of the very best systems for immunity to strong signal overload.

I have had repeaters since the 60's, including commercial sites. I prefer WACOM 4 cavity duplexers for my personal stuff. 4 cans are good to maybe 200-300 watts TX power with a reasonably clean TX.
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WB2WIK
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« Reply #8 on: July 17, 2011, 07:28:47 PM »

Wayne,

If you side mount both antennas on the same side of the tower you will barely see any differences. All you need to do is put the TX on the lower antenna and bump up power a bit.

If you use something like an old Motorola Micor you will be able to duplex with as little as 50 feet of vertical separation. Othere than a little TX composite noise it is one of the very best systems for immunity to strong signal overload.

I have had repeaters since the 60's, including commercial sites. I prefer WACOM 4 cavity duplexers for my personal stuff. 4 cans are good to maybe 200-300 watts TX power with a reasonably clean TX.


I preferred the WaCom duplexers also as they were BpBr (band pass, band reject) with two tunable elements per cavity, all of which took maybe five minutes to tune if you had the right equipment.

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K6AER
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Posts: 3535




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« Reply #9 on: July 25, 2011, 03:12:17 PM »

A 4 cavity (8 inch dia.) will give you about 80 dB if TX/RX isolation. With a reasonably clean transmitter and well tuned notches in the cavities you can run up to about 40 watts into the cavities.

If you put a isolator on the output of the transmitter be sure to insert a bandpass cavity to prevent isolator IMD from out of band transmitter sources. This is a problem with shared sites.

When you run power, 0ver 100 watts you will need a six cavity BPBR cavity system. Your antenna needs to be very close to 50 ohms. If the load is reactive from 50 ohms, this will shift the notch in the cavities closest to the load (antenna and feed).

In that you live in S. California and the repeater most likely will be on a mountain top I would ask the SCRBA technical committee for help. By the way how did you get coordination for a repeater on 2 meters?
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K4JJL
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Posts: 503




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« Reply #10 on: August 01, 2011, 08:01:51 AM »

"but they do not have to be modified - just retuned. "

That is, if you don't end up with a high split Mastr II.  Every Mastr II I've dealt with (VHF and UHF) was a high split and required 4-6 capacitor changes on the osc-mult board to get into the ham band.  This meant the difference between -50 dbm vs -120 dbm for 12 db sinad.

The VHF PLL exciters need capacitors and inductors changed on the bandpass filter board to get it to lock, as well.  Also, don't forget changing the aluminum tuning slug to ferrite.  I tried for a year to get by with the aluminum slug, but it was very unreliable.

The modifications are "baby food" to perform, but you will be really pissed if you think you tuned it right and the performance is that bad.

I usually end up having to "unwire" all the "bastardized customizations" on the card cage, the 10V regulator card, the audio card, and the TX control card to get it to work with an external controller.  This is where all the numbers inside triangles on the schematics are VERY VERY handy.  I try to utilize all the tone remote connections in order to take advantage of the built in noise filtering and trim pots for setting audio levels.

Jared
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K9KJM
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Posts: 2415




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« Reply #11 on: August 24, 2011, 09:59:13 PM »

The bottom line of if you use two different antennas or a duplexer is IF you have vertical tower space available to make it work, Along with an extra antenna and feedline.
One of our wide area repeaters uses different antennas, Receive at the very top of a 500 foot broadcast tower, Just above the FM broadcast antennas, And our transmit antenna just below the FM broadcast antennas, About 90 feet of seperation.   Works GREAT!   But we were very lucky to have this vertical tower space available to our club at really no cost, Along with the surplus 7/8 Andrew heliax to each antenna.
In most cases, It is just easier and cheaper to buy a high quality duplexer and go with one antenna and feedline.
(We are using four bay Telewave exposed dipole arrays)
I agree that some old used commercial repeaters can work well.  I have converted a number of Motorola Micors to the ham band with great results.   These things are seen at swapfests for really low bucks nowadays!
If you want to buy brand new, The Kenwood seems to be one of the better commercial repeaters  for the price (About 1K)
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W3DL
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Posts: 13




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« Reply #12 on: October 29, 2011, 04:59:51 PM »

I have been working on repeaters since the 70's both commercial and amateur. On 2 meters with the close spacing (600kHz) you need a lot of isolation to avoid receiver desense. Most of the VHF repeaters I have helped maintain have been 100 Watt transmitters. I have found the best configuration for trouble free operation is split antennas. With 6dB gain antennas we have always had plenty of isolation with 40' or 50' tip to tip vertical separation. That requires 60' to 70' space between antenna mounts for a 6dB stick.

With this setup and good shielding practice in the rack, the receiver never knows that the transmitter is on the air, regardless of the weather or anything else. The coverage is well matched with RX at the top, listening to an HT or 25 watt mobile, and 100 watts on the lower antenna.

With the 6 cavity duplexers we always had crackle or desensing when running 100 watts. Splitting the RX and TX antennas clears this up.
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K1ZJH
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Posts: 1185




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« Reply #13 on: November 12, 2011, 07:18:57 AM »

Simple bandpass cavities wouldn't due much when using seperate antennas. They don't
have enough selectivity. But, if you can find notch cavity filters they can provide about 30
dB of isolation at 600k kHz. Problem is the cavities aren't cheap. I've seen used two meter
diplexers selling for good prices these days as more machines are going of the air for lack
of activity.

Buy a good duplexer. I am partial to Sinclair products.  One other trick you can use is to
use split antennas, and a cheaper duplexer that is "spit" to provide even more attenuation
that it would provide with a single antenna. Whatever you do, the duplexer is the heart
of the repeater.

Pete
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WB2WIK
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Posts: 20666




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« Reply #14 on: November 14, 2011, 03:23:43 PM »

Whatever you do, the duplexer is the heart
of the repeater.


I always thought the heart of the repeater was the crazy person who would drive up a mountain at 2 AM to fix it.

That was usually me. Tongue
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