Leaderboard

I purchased two MXT575's. My mobile with the 6db midland antenna is working great for my needs. I can work all my local repeaters very reliably with my setup (20-30 mile range). And if I am stationary with a clear view of the north sky, I can check in on a repeater just over 90 miles from my location. Couldn't ask for much more with the MXT575 + 6db antenna. I particularly like the simplicity. Yes, I have a couple of Wouxon's where I can program in 1,000 channels. But honestly, all those channels turn into clutter. Let's face it, I am generally working 8 frequencies or less. I know some of you have multiple repeaters on the same freq with different tones - which has not been a problem for me - and from the map I look at around the country most do not have this problem. And I do enjoy checking in on other repeaters when I travel. But as mentioned above, I can make note on my phone of the repeaters I want to try. It takes 5 seconds tops to swap TX tones. I leave the RX tones open to listen for traffic. My second is for a base station. Still working on antennas for that one. I need more height at home to really get the performance I want. But I am able to check in on local repeaters. And it makes a very clean installation. I can hide away the main unit. I purchased a RJ45 extension cable (for mobile and base). With all the controls on the mic, the base unit and power supply are hidden. Very nice install. For my base installation, I even put a WIFI power outlet on the power supply. So when I am around town, if I want to contact Unit 2, I can turn on the base station from my phone, and call out to my base station from the mobile. The 575 returns to the last state, so powering on the power supply turns on the 575 to the last channel. Couldn't be happier with the 575.

Yes, there is. You are going to connect it up as if you are tuning it and look at the notch that shows up on the VNA. With one that's correctly tuned for a single frequency, the notch should be fairly tight, but you will be able to see the notch go down, and then come back up. The deepest part of the notch is the primary tuned frequency. You can also put markers on the close repeater frequencies and see what the difference in the notch value is. Obviously it will NOT be as deep as the primary tuned frequency but you can see what the numbers are. If the numbers are 3 dB or less between the center tuned frequency and the frequency of operation then you are gonna be safe to use it on that frequency. Now if you don't understand dB measurement then 3 dB doesn't sound like a lot. But it's 50% of the specific signal level. Lets say you are running full legal power of 50 watts. That is 47dBm. If you loose 3 dB of power it's now 44dBm. Thing is 44dBm is 25 watts. So a 3 dB change is NOT insignificant.

While I understand the negative comments aimed at the quality of Chinese duplexers that are sold on Amazon and eBay, I have built several inexpensive GMRS and digital voice amateur 440 repeaters using the Fumei and SGQ duplexers, and have achieved results comparable to using Phelps-Dodge and Combilent duplexers, which are usually a bit more money. These notch-type duplexers are a compromise, but for budget builds, they work ok. Like you, I had to start to digest the info that is out there, and I couldn’t afford the quality band pass-band reject duplexers, but we all have to start somewhere in the learning process, so these inexpensive Chinese duplexers serve a need. I ended up buying this cheap spectrum analyzer with tracking generator off eBay, so I could tune these duplexers…Tracking generator I also have a NanoVNA, but I found this SA w/tracking generator easier to deal with, although you have to search out the instructions, since it doesn’t come with any. Ok, I’ll save you the search…SA w/ tracking generator instructions Looking at the photos you posted, I would guess you don’t have things set up correctly. You will also need a 50 ohm dummy load connected to the duplexer port you are not adjusting, and with the duplexer you have, you should be able to get to -85dB or greater. Use this site to figure your loss through the duplexer …dB calculator Using this calculator, and measuring the output from the transmitter radio, and measuring the output of the duplexer, I generally achieve about a 1.3-1.5 db loss through the Fumei/SGQ/Combilent duplexers, which is average for these types of duplexers. I am not a professional. I am not someone who lives and breathes this stuff. I am just a hobbyist with decades of experience in blowing things up, shorting things out, and otherwise screwing the pooch with electronics. I am just giving you some info that works for me, after building 4 or 5 GMRS repeaters out of old commercial mobile radios, and 3 multimode (DMR/C4FM/D-Star) amateur repeaters. Are there better ways to do it than I did ? yeah…do I care? no. The repeaters I built with these types of duplexers still work, don’t cause issues because they are not in RF-intensive locations (well, actually, one is at a county EOC and works great), and serve the users’ needs at an affordable cost.

The best advice I can give you is to download and use the free software: https://nanovna.com/?page_id=90

Can you post a pic of your test setup and change your span to 10mhz centered on 465? The dynamic range on a NanoVNA isn't great but you should be able to see down to -70dB somewhat OK. A notch at -22dB is barely anything and indicates something's amiss.

This guy looks at the characteristics of a duplexer using two different NanoVNAs in this video. In this next one he tunes a duplexer:

Yeah, I am glad we figured that out. And I hope the guy finds a location and gets a repeater on the air. I actually went back and read the first post and of course, NONE of what I was talking about was in there. So I went down and found the post I was referring to. I sort of figured it was something like that, since what you were saying didn't at all fit with what I was talking about. I am really glad I did that so we could get clarification and on the same page. I hate seeing stuff like that. And unfortunately it's more common than folks might think. Since we were discussing MOU's. I am actually getting ready to work on one with the county I live in for two repeaters I am housing on the tower. Both are ham repeaters that will be general amateur use unless there is a specific need for EMCOMM. I have a unique situation with the tower location and coverage foot print that allows access from both my county of residence and the county to my West that has the state capital and the State EMA office that is almost LOS to my tower. The repeaters will be running from a battery plant and rectifier (150 AH will be the initial storage capacity) that will hopefully be solar and wind fed along with grid to the rectifier. I plan on growing the battery plant as funds become available and getting it to at least 300Ah at 48 volt.

I just got a 1486 last week and I cut it for 462 using a nano-VNA. The sections that need be to cut look like they are brass. I cut them accurate and clean just using my scroll saw So far performance is pretty good. The chart was pretty accurate for me.. In the end my lengths were exactly what the chart suggested for 462. I have heard that many people found that the chart was off for them so be careful not to cut too short! It is easy to be off by a mm or so depending how you hold your ruler to the base of the coils. I measured with the butt of my zero clearance ruler touching the base of the coil. .

I have a Tram 1486 and i tuned mine because the instructions said I had to - following the instructions to cut both the top and bottom sections, I tuned it to 467.700 because it is for a repeater... But when I checked the tune on 462.700, the SWR was still a very low - dont remember exactly what it was, but under 3:1.

Never mind. I'm a dumbass. ----------------------------- I have a question... why are you going to try to tuning it? It's a stacked 5 phase element high gain antenna with the center frequency being very close to GMRS frequencies. Due to being a stacked phased element antenna (5-1/2l), if you cut the top or bottom, you are only adjusting one element and throwing the phasing completely out of whack, causing massive issues with receive and transmit, regardless of what the SWR meter says. You can't even adjust all 5 elements because the LC network of the antenna is tuned for factory resonance. I would just leave it alone.

That's a sad post to read; but I also think it's great that they posted the story directly and cleanly for the community to digest; it's disappointing that they weren't able to get the support they needed. Many don't realize the hard work and costs (be it direct or merit based) it takes to put up a good repeater on a good site; it's not just the hardware costs, it's the maintenance of the gear, it's the work to earn your keep there or pay the bill or both, it's the fuel to drive to these places which can be hours away, it all adds up both financially and in terms of time. These machines are ubiquitous, such a tremendous part of people's every day use yet in the background so that it's not realized that someone has to put them up and keep them up, someone has to pay for them in one way or another, someone has the balance their work/family time to look after them, and yet they're often made freely available to the community because it provides a public service for the greater good. They were assessed $125/month to be on a tower covering the bay area and then some; in reality this in itself is an amazing rental rate for a site covering small city let alone the bay area! Support your local GMRS groups! It's hard, time consuming, and expensive work to provide these machines. If you can't do this financially; help pull some weight in other ways - step up as net control operator, help out newcomers, buy someone a beer, and if for whatever reason you can't do that, at least be courteous and don't complain that you can't receive it S9+60dB on a Boafeng in your mom's reinforced concrete basement 80 miles away.

Oh, wow! Yeah, I did not see that post. That seems very sketchy, even if it's a private business. I fully agree with you in that context... I appreciate the info. No worries. I'm glad we were able to figure out the disconnect. It was a harmless mix-up on both our parts. I'm glad everything stayed civil while we discussed the topic. I know it seems like a strange thing to say, but that doesn't happen often when people are in disagreement on the internet (pretty sure I am guilty of that myself, recently) . It makes it much easier to continue to enjoy the conversation.

A typical "notch" style compact duplexer (the ones you usually see advertised at prices around $100) can have a really wide and lousy tune job applied - and will "cover" the whole GMRS band, but they won't do it very well. If you're OK with having 3-5 dB of desense - then yeah, go for it. It will halfway kinda work as long as you've got a strong signal into the repeater.

I only watch the ones with GMRS geeks and youtube hobos.

All of the good Youtube GMRS channels will be publishing reviews on the day it's available for purchase..