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I have a cheap spectrum analyzer, but I have been hoping someone would test the RT97 duplexer with a quality analyzer and post the results. Since that hasn't happened yet, I decided to bite the bullet, and do it myself. Hopefully someone with a quality instrument will see this and add to it.

A bit of background first. I ordered my repeater configured for 46X.550. I received it a month ago, but haven't had time to play with it until the Memorial day weekend. That weekend I did a field test, with a dipole on a 20 foot portable tripod mast. I was pleased to see a solid 5 watts out, with swr very close to 1.1. 

I then set my mobile to 5 watts and started to drive. I expected to get about 8 to 9 miles range, here in the flatlands of Southwest Florida, and I was pleased with the results. In one direction I went to an area which lies a few feet lower than the repeater location, and went nearly 12 miles before the S meter dipped below 2 and began to flutter.

 

I took out the duplexer and attached it to the cheap analyzer I got for $100 via Ebay. The low side notch, supposed to be at 462.5500 Mhz, was actually at 462.7010, as shown in the first photo.

462A.png

The tracking generator of this analyzer is about -30 dBm, so this shows an attenuation of 87 dB at the notch. Not bad for such a tiny duplexer, but at the desired frequency there was only 71 dB attenuation. After I tweaked it, I got the notch centered on 462.5500 at an attenuation of 80 dB, shown in the next photo.

462B.png

Next up I looked at the high side, where the notch was supposed to be at 467.5500 Mhz. It was actually at 467.4896, with 83 dB attenuation as shown below.

467A.png

A little tweak gave me 80 dB attenuation at the desired frequency.

If I get the time this weekend, I'll do another field test with the tweaked duplexer and comment on here with the results.

 

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Be careful with the fine tuning of the duplexer, you'll get it more accurate on the notch but you'll also end up increasing the insertion loss and the RT97 will put out less than 5W.

Seems it's a 15W transmitter but the duplexer loss is folding back to 10W if you have the frequencies far enough apart or 5W if you're in the 5 MHz split range. If you get obsessive about the notch you might find out you're putting out 1-2W because of the insertion loss (that lower curved line).

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1 hour ago, rdunajewski said:

Be careful with the fine tuning of the duplexer, you'll get it more accurate on the notch but you'll also end up increasing the insertion loss and the RT97 will put out less than 5W.

I thought you wanted the notch dead on for duplexer's to prevent desense? What causes the increased insertion loss?

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I have used my el-cheapo analyzer to retune a few mobile duplexers in the past with good results, though none as small as this one. It is about 1/3 the size of other duplexers I have tuned. The trick is to be careful, take it slow and easy, and use plastic tweaking tools (I inherited a nice set from my grandfather). 

I did retest the RT97 after this tuning, in the shack. Still 5 watts out, and still near 1.1 swr ( I have an old analog swr meter, so judge accordingly). When the storms clear up here (probably tomorrow morning), I will be putting it back on the mast for further testing... I know that what looks good in the lab doesn't always work well out in the real world :).

 

 

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2 hours ago, WRAK968 said:

I thought you wanted the notch dead on for duplexer's to prevent desense? What causes the increased insertion loss?

You do, but the insertion loss is caused by the fact that the "V" notch trough isn't the only factor. The insertion loss comes from what the filter is doing to the pass frequency, which on these small units is going to be much higher at a 5 MHz split than a 10 MHz split. So while you're filtering out 462.xxx MHz in the notch, you're also losing power on 467.xxx MHz due to the curve to the right. Basically you're too close to the notch, and you're losing some power there. This is also why the duplexers are marked with a high and low side, as the configuration of that curve to the left or right depends on which side you're on.

Normally the insertion loss is small, like 1 to 1.5 dB on a good duplexer, but you don't have that low of curve to contend with in those cases. So to avoid this, you can either tune the V lower in frequency (which isn't great, as you're losing isolation) or you can try to make it a little shallower as long as you're still getting adequate isolation. Since it's a low power transmitter, you don't need -90 dB but the more isolation, of course, the better. It's just a balancing act to notch as much as you can without negatively impacting the frequency you want to pass.

I had to retune the duplexers that came in an engineering sample I received of these repeaters, as they were tuned for 453 and 463 MHz. For one, I had to swap the high and low sides since they were set up with RX on 453 and TX and 463, which was an issue with the short SMA jumpers they included. Then, I had to retune the duplexers to the GMRS band and be careful to not impact the pass side too much.

Keep in mind I'm no expert, my background is software not RF design or Electrical Engineering. I know just enough to be dangerous and convince myself I'm right most of the time. 🤓

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I would rather be obsessive about maximizing reject notch as T-R isolation is the main goal of a duplexer. If you can keep the insertion loss in spec and not excessive, fine, but insertion loss is an expected feature of a duplexer. There is no free lunch.

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23 hours ago, WRAK968 said:

I see what your getting at now, kinda interesting and I never thought of it that way :)

This is why professional duplexers used in both ham and GMRS repeaters are so expensive!

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On 6/4/2021 at 6:05 PM, JLeikhim said:

I would rather be obsessive about maximizing reject notch as T-R isolation is the main goal of a duplexer. If you can keep the insertion loss in spec and not excessive, fine, but insertion loss is an expected feature of a duplexer. There is no free lunch.

You're right, but in the case of these small duplexers you can't just worry about the notch or you'll end up with a dummy load. The insertion loss is SO bad at 5 MHz split that you need to consider it, or you're burning 15W to get maybe 1W out of the transmitter (and your loss is also on the receive side, which isn't doing you any favors). So, it's really a balancing act between both of them. Filter the bad, but try to keep the good. Filter both, and what's the point?

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TODAY I put the retuned RT97 back on the 22 foot mast, and repeated my drive from memorial day weekend. There really was little difference. It did seem that there was less hiss, but that is entirely subjective.

So, my take on it is... my unit arrived properly tuned for 46X.550 as requested. Granted that I made only very slight tweaks, but no doubt that their alignment equipment is much better than mine.

Overall it has met my expectations. Though I do have some quibbles.

If it had at least a minimal morse code ID at selectable 10 or 15 minute intervals, it's appeal to radio amateurs and GMRS users would vastly expand. I really can't believe their marketing people missed such a simple to add feature. There IS room in the case for an Arduino nano, or similar sized board. So (after hurricane season) I plan to open it up and check out the signal paths, with the goal of making a minimal ID board. 

Another quibble, as I mentioned before, is the apparent vulnerability of the LCD display board. Other than that, the RT97 is built like a tank.

My final quibble is the lack of a convenient way to mount the thing. I'm still trying to figure out an easy way.

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No requirements to have morse code id on a repeater. As long as user ID's.

The box is made to hang on a line between telephone poles so you can get some clamp style kits for it but for what GMRS users are doing i agree.

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4 hours ago, oldtech said:

My final quibble is the lack of a convenient way to mount the thing. I'm still trying to figure out an easy way.

A piece of wood, four screw eye hooks and two bungy cords... 😉

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9 hours ago, oldtech said:

TODAY I put the retuned RT97 back on the 22 foot mast, and repeated my drive from memorial day weekend. There really was little difference. It did seem that there was less hiss, but that is entirely subjective.

So, my take on it is... my unit arrived properly tuned for 46X.550 as requested. Granted that I made only very slight tweaks, but no doubt that their alignment equipment is much better than mine.

Overall it has met my expectations. Though I do have some quibbles.

If it had at least a minimal morse code ID at selectable 10 or 15 minute intervals, it's appeal to radio amateurs and GMRS users would vastly expand. I really can't believe their marketing people missed such a simple to add feature. There IS room in the case for an Arduino nano, or similar sized board. So (after hurricane season) I plan to open it up and check out the signal paths, with the goal of making a minimal ID board. 

Another quibble, as I mentioned before, is the apparent vulnerability of the LCD display board. Other than that, the RT97 is built like a tank.

My final quibble is the lack of a convenient way to mount the thing. I'm still trying to figure out an easy way.

I have already looked into it, but there are no PTT signals anywhere inside. They use serial communications like I2C to command the radio IC to transmit, rather than use a logic signal. Without some serious, serious hacking, there's no way to add an ID board or external controller to these.

However, I approached Retevis about this and they are making a version with an external connector on it. I have received the engineering samples and they work, although we may ask for some tweaks. Hoping to sell these on the store soon. We'll be able to sell an all-in-one linking bundle that is completely plug-n-play for the myGMRS Network.

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6 minutes ago, rdunajewski said:

I have already looked into it, but there are no PTT signals anywhere inside. They use serial communications like I2C to command the radio IC to transmit, rather than use a logic signal. Without some serious, serious hacking, there's no way to add an ID board or external controller to these.

However, I approached Retevis about this and they are making a version with an external connector on it. I have received the engineering samples and they work, although we may ask for some tweaks. Hoping to sell these on the store soon. We'll be able to sell an all-in-one linking bundle that is completely plug-n-play for the myGMRS Network.

That's great they are working on at least providing a 'hook' for ID and/or external controller. I believe it will greatly increase the appeal. I do not need it for my own use, it's family only. 

 

5 hours ago, n4gix said:

A piece of wood, four screw eye hooks and two bungy cords... 😉

That's basically how I have it mounted now! B)

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  • 2 weeks later...
On 6/6/2021 at 8:54 PM, rdunajewski said:

I have already looked into it, but there are no PTT signals anywhere inside. They use serial communications like I2C to command the radio IC to transmit, rather than use a logic signal. Without some serious, serious hacking, there's no way to add an ID board or external controller to these.

However, I approached Retevis about this and they are making a version with an external connector on it. I have received the engineering samples and they work, although we may ask for some tweaks. Hoping to sell these on the store soon. We'll be able to sell an all-in-one linking bundle that is completely plug-n-play for the myGMRS Network.

Really, if they added a dedicated jack for the off-the-shelf ID-o-Matic board, that'd solve like every single problem in one fell swoop.

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