Repeater amplifier

[motolover]

I have a gr1225 repeater and they guy i bought it from said he could not put it any more then 30 watts becuase of the duplxer. if i bought a amplifer putting out 50 watts and put it on the feedline to the ant. would this work ?

 

[fineshot1]

More power alone is not the answer you seem to be seeking. I must assume you are not happy with the coverage of the repeater. Your $ is better spent on a better location for the repeater or better feedline/antenna combo. The difference of 30W and 50W is a waist of $ and will probably not be noticeable, however you have not provided any info on any of the parameters needed for a professional to tell you much.

 

[n2mdk]

It's been quite a while since I worked around repeater, amateur at that, but what I believe he was saying is that 30W is the max output before the transmit will interfere with the repeaters receive. I'm sure someone else will chime in here, but that's what I think was being said.

 

[motolover]

Its a gmrs repeater on a 100 ft tower running 30 watts

 

[n2mdk]

It really depends upon the terrain and structures around the area. If 100ft is higher than most of the surrounding area it should be fine. What you have to understand is that the receive on a repeater is really more important than your output power. If someone 30 miles away can hear the repeater, but it can't hear them then then what good is all that output power.

 

[kb2vxa]

Let's just cut to the short answer, no. Without getting tangled in a technical discussion another short answer is you'll not notice the slightest difference between 30 and 50 watts.

 

[n5usr]

Putting the amp in the feedline from the duplexer to the antenna means your receiver won't have anything to listen to. Due to how an amp works, you can't listen through it at the same time you transmit (otherwise, its own output would feedback to its input and the amp would self-destruct).

So putting an amp there will only have the effect of letting people "kerchunk" your repeater - as soon as the receiver sees their signal, the transmitter kicks in, switching the amp on and the receiver goes deaf (since it effectively has no antenna) until the transmitter drops.

If the duplexer is a problem, I would either make sure it's properly tuned or get another one. UHF duplexers are generally quite easy to come by and not terribly expensive since they can be so small. Certainly you can get one that doesn't limit your power output that low.

 

[motolover]

thanks guys

 

[zz0468]

...they guy i bought it from said he could not put it any more then 30 watts becuase of the duplxer.

Perhaps he means that the duplexer doesn't have enough isolation to be able to handle more than a 30 watt transmitter. Running more power would then just desense the receiver. If that's the case, an amplifier would make the situation worse. A better duplexer might be what's required. The fact that he mentioned the duplexer might be important...

What are you trying to accomplish that 30 watts isn't sufficient?

 

[FerSomma]

How to obtain more coverage area

Hello, thread, if you want to extend your coverage area, there are many things you have to have in mind, not only the output power. Is your 30W amplifier clipping the signal??, or the repeater has an APC or ALC or AGC system that reduce your gain??? You don't have to assume that if you have a 30W amplifier then the output power will be 30W..... it depends on its gain and input power.. if your 30W amplifier is working at 10W and you install a 50W amplifier with the same gain, then your output power will still be 10W.. in this case, you need more gain or input power to drive the power amplifier into its max power and that is thats way to expand your coverage area instead of replacing the 30W amplifier.. it is true that you have a potentially bigger coverage area with a 50W amplifier that with a 30W amplifier, but first you will have to carefully obtain the necessary conditions to make it work properly.

Regards.
www.viewrd.com.ar

 

[FerSomma]

Output power versus Duplexer

It is not true that a repeater will have isolation problems on its duplexer tray because of changing the max output power of the final stages. The aspect that you have to have in mind here is if the gain of the new amplifier is higher than the previous one. What you have to maintain is the balance between amplifiers gain and duplexers isolation. If changing that amplifier do not produce an increment on the power gain, then you won´t have any problems if you replace it, however, the only reason to make such change must be because your actual amplifier is clipping the signal and compressing it´s output power.. if your 30W amplifier is not clipping the signal you will have to review your antenna array to increase the reception, otherwise, if you just change the amplifier for a bigger one without driving it into it´s max output power, you will only get less efficiency and more power consumption.

Regards
www.viewrd.com.ar

 

[prcguy]

I think the original poster wants to up his power to 50w, not replace his 30w amp with a 50w version running at 30w. All repeater amps run (or should run) at or near saturation or "clipping" if you want to call it that. These are class C amps not linear amps and many will get unstable if the output power is backed off more that about 50%. A given duplexer will only have a certain amount of TX-RX isolation and the original poster is probably using a small mobile duplexer with 60-70dB max isolation. These duplexers also have a maximum power rating of 30-50w and it you exceed this they can arc internally, not to mention there may not be enough isolation to duplex without desense.
As others have mentioned, going from 30w to 50w will probably go unnoticed and unless you upgrade the duplexer, your receive may degrade.
prcguy

It is not true that a repeater will have isolation problems on its duplexer tray because of changing the max output power of the final stages. The aspect that you have to have in mind here is if the gain of the new amplifier is higher than the previous one. What you have to maintain is the balance between amplifiers gain and duplexers isolation. If changing that amplifier do not produce an increment on the power gain, then you won´t have any problems if you replace it, however, the only reason to make such change must be because your actual amplifier is clipping the signal and compressing it´s output power.. if your 30W amplifier is not clipping the signal you will have to review your antenna array to increase the reception, otherwise, if you just change the amplifier for a bigger one without driving it into it´s max output power, you will only get less efficiency and more power consumption.

Regards
www.viewrd.com.ar

 

[mjthomas59]

Unless the OP either A) doesn't have a amp at all, or B) his amp quit working on him, then buying the bigger amp isn't worth the money. I can assure you that 20 watts isn't a whole hill of beans in regards to coverage area on UHF.

The height of your antenna is the most critical element. GMRS radios are only rated at 5 watts, so while yes your repeater may be heard(received) at 30 miles, no one will be able to respond(transmit) and hit your repeater from that distance.(i'm just pulling out random numbers, its the principle thats important)

I'd be more interested in getting the maximum out of what you have, and that is ensuring you have a top quality antenna if you don't already, and running the best feedline you can afford, you'll see more improvement from that than anything else... other than more antenna height that is.

 

[awall]

The best thing you can do to improve your repeater operation is to put in a good quality reciever pre-amp. If you double your output power you will only see about 2 or 3 miles extended range and it will not do anything for the reciever. Another trick that works real well is to get rid of the duplexer and run 2 antenna's and feed lines. Put your recieve antenna on top and hang the transmit antenna upside down about 20" or so below the recive antenna. Depending on the quality of your duplexer, you may have up to 2db loss of transmit signal so removing it would give you much better erp. By doing this (depending on terain) you could easilly double your range.

 

[zz0468]

The best thing you can do to improve your repeater operation is to put in a good quality reciever pre-amp.

I disagree. The best thing you can do is eliminate any desense so that the receiver performance is identical with the transmitter (all transmitters at the site, really) on or off. Adding a really hot preamp can make it impossible to fully eliminate desense.

If you double your output power you will only see about 2 or 3 miles extended range and it will not do anything for the reciever.

That's a gross over simplification, and not necessarily true, depending on the power levels involved. It follows the inverse square law, where double/half the distance involves a 6 db change in power level. That applies in free space, so terrain is going to have an influence.

Another trick that works real well is to get rid of the duplexer and run 2 antenna's and feed lines. Put your recieve antenna on top and hang the transmit antenna upside down about 20" or so below the recive antenna.

This is a useful technique, although the upside down antenna has some caveats. Not all antennas are "upside-down friendly", meaning there may be moisture/drainage problems. Or perhaps the radiation pattern is all wrong when it's upside down. But separate antennas are a useful technique in some, but not all , circumstances.

Depending on the quality of your duplexer, you may have up to 2db loss of transmit signal so removing it would give you much better erp. By doing this (depending on terain) you could easilly double your range.

This is in contradiction to both your statement about doubling the power, and the inverse square law. 2 db of duplexer loss is not necessarily unreasonable, and being considerably less than 6 db doesn't come close to halving the usable range. The trick to dealing with this is to start with enough transmitter power so that after 2 db loss, you have a satisfactory amount at the antenna port. It's all part of the gain/loss budget you do when you design your repeater system.

 

[prcguy]

You still need cavity filters if you run separate antennas, even with 30ft vertical height between the antennas the most isolation you can get is about 30dB. A good duplexer that's tuned properly will have about .75dB loss with 80dB or more isolation. The inexpensive mobile duplexers can have up to 2dB loss. Running separate antennas with two cavities per leg will have about .5dB loss and the transmit antenna will usually be lower, reducing coverage. The best use of separate antennas is for a master receive system for multiple repeaters. If you add a preamp you want additional filtering to give at least as much isolation at 5MHz spacing as the preamp has gain. Lots of things to consider before tinkering with your repeater.
prcguy

The best thing you can do to improve your repeater operation is to put in a good quality reciever pre-amp. If you double your output power you will only see about 2 or 3 miles extended range and it will not do anything for the reciever. Another trick that works real well is to get rid of the duplexer and run 2 antenna's and feed lines. Put your recieve antenna on top and hang the transmit antenna upside down about 20" or so below the recive antenna. Depending on the quality of your duplexer, you may have up to 2db loss of transmit signal so removing it would give you much better erp. By doing this (depending on terain) you could easilly double your range.