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Retevis RT97S Repeater and RF Power Amplifier


lawenforcement
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I recently purchased a Retevis RT97S Portable Repeater through the Mygmrs.com website. After viewing products on the Retevis website I noted that they sell this repeater in a package that includes an external RF power amplifier that's installed between the repeater's output and the antenna feedline in order to increase the repeater's output wattage. Does anyone know how this configuration could possibly work? It seems to me that once the RF power amp is keyed up, it would cut off the input to the repeater / duplexer. The power amp can't possibly be passing the incoming frequency to the repeater / duplexer's antenna input while the external power amp is keyed up so the repeater's receiver would effectively be without an antenna connected during transmission. Retevis responded to me by saying they have customers who are successfully using this product combination  BUT they did not say they have tested it themselves. 

 

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I was stupid to believe I could use an amp (as i've seen on youtube...but with a different duplexer.)

I wish I found this thread last week. Thankfully I got the rt97 and amp on Amazon so I can make an easy return...honestly I got it there initially to experiment and return if it did not work out.

I just purchased the RT97S on there since I want the ID O matic for features outside of station ID.

Being ignorant when I installed the amp (yes this is wrong after thinking and reading this thread)..weird things happened when outside of my home with the antenna is 30 feet or so above my roof. My jpole antenna was transmitting as a beam east to west but receiving omni....the other TX direction was either noise or nothing at all. I was not able to replicate the issue with the amp removed.

If my 5 watt HT can be picked up 10-20 miles (depending on direction) away from my home station then it's good enough. What does 50 watts do for me...from a signal that was great..to slightly better. If I want more distance i'll use the commercial GMRS repeater in my area, connect an HT to Zello, or build one. Building one is like an oil change, I can do it for cheaper and with better parts but I rather pay someone to get theirs hands dirty.

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1 hour ago, HrXkb3UjmgFvBe said:

I was stupid to believe I could use an amp (as i've seen on youtube...but with a different duplexer.)

I wish I found this thread last week. Thankfully I got the rt97 and amp on Amazon so I can make an easy return...honestly I got it there initially to experiment and return if it did not work out.

I just purchased the RT97S on there since I want the ID O matic for features outside of station ID.

Being ignorant when I installed the amp (yes this is wrong after thinking and reading this thread)..weird things happened when outside of my home with the antenna is 30 feet or so above my roof. My jpole antenna was transmitting as a beam east to west but receiving omni....the other TX direction was either noise or nothing at all. I was not able to replicate the issue with the amp removed.

If my 5 watt HT can be picked up 10-20 miles (depending on direction) away from my home station then it's good enough. What does 50 watts do for me...from a signal that was great..to slightly better. If I want more distance i'll use the commercial GMRS repeater in my area, connect an HT to Zello, or build one. Building one is like an oil change, I can do it for cheaper and with better parts but I rather pay someone to get theirs hands dirty.

JPoles are not omni-directional, they have a deep null and some gain in the other axis. https://www.w8ji.com/end-fed_vertical.htm

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42 minutes ago, tweiss3 said:

JPoles are not omni-directional, they have a deep null and some gain in the other axis. https://www.w8ji.com/end-fed_vertical.htm

Sure I understand, I did not explain it properly. BTW, the link is interesting and perhaps the cheapo amp changed the shape for the worse given that's it's not omni.

I was driving around my block (the issues started around 5-10 seconds away from my home) where the sound (not signal or wave) was 100% to my ear before the repeater and amp was ever in use.

I was using my station on simplex with my family for the past year without issues with any nearby direction. Again, I was only a few blocks down the road when the signal was warped because of my noob experiment. The outcome is interesting, learned a lot by my own ignorance.

I ordered a cheap omni last night to see what happens and to keep it as a backup down the road. jpole was what I had for GMRS and it's clearly not intended for repeater use with an amp incorrectly used. My dual bander amateur worked great without the amp...but I need a quality tuned GMRS base antenna. I have parts to build one..but I don't want to , I just want to toss up a completed antenna without tuning it.

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2 hours ago, tweiss3 said:

JPoles are not omni-directional, they have a deep null and some gain in the other axis. https://www.w8ji.com/end-fed_vertical.htm

I did a simple J-Pole simulation a while back. The results I got don't look like the one in the link. However the pattern isn't symmetrical because the antenna isn't symmetrical due to the 1/4 wave matching stub. I wonder just how he modeled the feed line since the software doesn't include it as part of the radiation model that I can see. If it did you should have seen it as some kind of "long" wire segment. Typically the software simply does the impedance transformations and time delay calculations, then applies those to the ends of the transmission line in the model. The line itself is assume to be non radiating. At least that's my understanding in the software I use.

J-Poles are difficult to model correctly as it is, and that's mostly because of the 1/4 wave matching section.

J-Pole.jpg

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On 1/22/2022 at 9:48 PM, lawenforcement said:

I recently purchased a Retevis RT97S Portable Repeater through the Mygmrs.com website. After viewing products on the Retevis website I noted that they sell this repeater in a package that includes an external RF power amplifier that's installed between the repeater's output and the antenna feedline in order to increase the repeater's output wattage. Does anyone know how this configuration could possibly work? It seems to me that once the RF power amp is keyed up, it would cut off the input to the repeater / duplexer. The power amp can't possibly be passing the incoming frequency to the repeater / duplexer's antenna input while the external power amp is keyed up so the repeater's receiver would effectively be without an antenna connected during transmission. Retevis responded to me by saying they have customers who are successfully using this product combination  BUT they did not say they have tested it themselves. 

 

I was wondering about that scenario a couple days ago.  It says on the Baofeng 40-watt amp description that the analog models pass RF passively when the amp is off, and I was wondering if I could get away with a cheater repeater like this.

 

Short answer?  Not without replacing the duplexer with a fifty-watt model, sounds like.  That's another hundred bucks on Amazon…

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27 minutes ago, TNRonin said:

Excellent thread! What would be the best Omni directional antenna? I have a j-pole I'm about to install, and testing will show if it works well enough I'm sure.
 

The choice for a repeater antenna depends on how deep your pocketbook is. The better antennas are all large and have gain measured in dBd. DBd gain is a multiplier over the power radiated from your unity gain J-pole. If a gain figure is published using dBi, subtract 2.1 from the figure to get the true gain of the antenna. So, a 2.1 gain dBi antenna actually has zero or no gain over a half wave dipole. Considered as the best all-round antenna is the CommScope DB 420-B radiating over 8 times more power than its input. 

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The choice for a repeater antenna depends on how deep your pocketbook is. The better antennas are all large and have gain measured in dBd. DBd gain is a multiplier over the power radiated from your unity gain J-pole. If a gain figure is published using dBi, subtract 2.1 from the figure to get the true gain of the antenna. So, a 2.1 gain dBi antenna actually has zero or no gain over a half wave dipole. Considered as the best all-round antenna is the CommScope DB 420-B radiating over 8 times more power than its input. 
Ya pockets are not that deep. Any other suggestions?

Sent from my SM-T860 using Tapatalk

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1 hour ago, TNRonin said:

Ya pockets are not that deep. Any other suggestions?
 

The cost of the antenna also includes the mast/mount and feedline. Start with your budget and then the feedline needed before deciding on the mast and mount. That will tell you what's left for the antenna.

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I have a thread here with various antenna solutions - 
 
 
Remember you get what you pay for. Manage expectations. A cheap antenna and poor feedline will sour your taste in GMRS and waste funds. 
Gotcha. I have a lm400 feed line. Just trying to get something in my budget.

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Understand that LMR 400 is not good when it gets moisture in it for repeater use. If your going to use it make it as short as physically possible and make sure to seal properly the connections. Many thread on duplex use of LMR and why not to use it. If this is for a temporary situation it can work OK. 

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I am going to throw some knockoff LMR400 (KMR400) I have laying around on my RT97. This will be an outdoors installation. Just to see what happens.  There are all these posts saying not to use it for theoretical reasons but I have yet to find a post where one did use it and came back with real world results. I have been using RG400...and that has been fine but I wanna try the LMR for the heck of it.

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There are many posts with the issues with LMR style cable and the interference it will cause in itself. In the end it performs poor. As someone who has been in the commercial/public safety world work for 30+ years I have seen it over and over. I try to help folks not spend funds on stuff that they will replace in the end. LMR is ideal for base stations and any simplex operation, but in duplex operation especially in the UHF band it will suffer. I run it on all my control stations at home but anything with a repeater I use different cables. 

The RT97has the advantage of a waterproof case. The best install is a short LDF jumper to to the antenna and mount the actual repeater on the mast. Run DC to the repeater. That is how 90% of the cell sites out there as the radio is within a foot or 2 of the panel antenna. A 2' piece of LDF40-50 would be ideal. Just another option. 

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There are many posts with the issues with LMR style cable and the interference it will cause in itself. In the end it performs poor. As someone who has been in the commercial/public safety world work for 30+ years I have seen it over and over. I try to help folks not spend funds on stuff that they will replace in the end. LMR is ideal for base stations and any simplex operation, but in duplex operation especially in the UHF band it will suffer. I run it on all my control stations at home but anything with a repeater I use different cables. 
The RT97has the advantage of a waterproof case. The best install is a short LDF jumper to to the antenna and mount the actual repeater on the mast. Run DC to the repeater. That is how 90% of the cell sites out there as the radio is within a foot or 2 of the panel antenna. A 2' piece of LDF40-50 would be ideal. Just another option. 
What the alternative?!?

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TNRonin,

Youre getting good advice from radio professionals about the best antennas and feed line. That’s great if you really want the best. If that’s the case simply ignore the rest of this post. 
If you’re still reading you might be a hobbyist who’s just interested in “good enough“ and the financial savings that implies. 
You already have LM400 feedline. 
I think you already have a J-pole. 
A short run of LM400 will have nearly no discernible incremental losses compared to hardline. Just make sure you seal the connectors against moisture. 
A better antenna will make a difference, but how much is that worth to you?  
Hook up what you have and try it. It won’t cost you anything but your time and I guarantee you’ll learn from the experience. Then, if it doesn’t work like you want, you can always try a different antenna (or perhaps raising the j-pole). You can always upgrade the feed line as well. 
I would hate to see your enjoyment of the hobby diminished because you’ve been convinced that what you have isn’t good enough when the actual incremental improvement is only measurable to professionals. 
Don’t let the best stand in the way of the good enough. Lots of people use LM400 and j-poles with inexpensive repeaters and do just fine. In the end elevation and location are the most important factors and all the money you have cannot overcome that. 
 

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I've been using LMR400 on my repeater for over a year-and-a-half and it has been plenty "good enough" for me and everyone in the several hundred square miles that the repeater covers.

Is it as good as a multi-thousand dollar commercial-grade repeater installation?  NO

Will the coax last forever? NO

Does it work? YES

Be wary of "some people" that love spending other people's money.

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The issue in the end is not money or lack there of. Its location. If your repeater is on a 5000' mountain then inferior equipment is made up for height. Most folks are installing a repeater on a garage or house in town. IF you look at the true loss of cables and adapters you will see noticeable differences and may sour the hobby taste. Very few folks are on sites that have LOS to hundreds of square miles but I guess on youtube anything is possible. 

The repeater in question is about 5-7 watts leaving the antenna port. 25' of LMR at 460 is about .75 db of loss. Now add .50db for each adapter you use. Your rapidly up to 3db which is about 1/2 your power. So when your repeater on your garage is only transmitting 2-3 watts then folks ask why they handheld inside the house can talk to another and not thru the repeater. 

But I digress I keep forgetting the hobby is about doing it as cheap as possible as hams do in this new age. BTW you can grab 1/2 LDF jumpers on ebay for under $50 if you do some searching. I just sold 6 1/2" commscope superflex jumpers for $30 each with N (M) on both ends. So you can find some decent stuff if you look. 

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