Lscott

That's a nice write up on preselector filters. For anybody who has an issue with too much bandwidth on the front end you can buy a cheap bandpass filter, 0.6 MHz to 8 MHz range, to put inline between the radio and the antenna. The rejection for out of band signals is petty good at 50db. I think the vendor will tune the filter for you before shipping. http://www.409shop.com/409shop_product.php?id=125434

No it's not a sarcastic comment, just an observation. I've occasionally been guilty of the same thing, blaming lack of filtering or technical short comings on "intermod". I wanted to point out there are more reasons for poor selectivity or interference. Understanding what the underlying nature of the problem may suggest a possible solution or what radio specifications are really important. You soon learn that power isn't everything, and a bad or poor RX'er design can break a radio system. If you can't hear them what's the point? Looking at the typical manufacture's spec's for many radios most never mention the above points, and with analog radios specifically the image rejection. The ARRL lab does a fairly good job of testing radios when they publish a review. You have to dig through the test results for the info. The sad part is they can't test everything out there, and it could be months or even longer before they do a test and review, if ever, on a particular model. Most Ham radios have wide open RX front ends because people want to use them as cheap scanners. I use many of my radios that way, Ham and commercial. While that maybe appealing however that means a compromise in other areas. One reason why the commercial radios mostly perform better in high noise and RF heavy environments is the RX'er design. The radios are generally single band with just enough front end bandwidth to cover the commercial section of either the VHF or UHF band they were designed for and no more. The forum is a place here to trade information. I like to includes links in many of my posts for background that saves a lot of typing. Why repeat the same information when somebody else has already done the job, and likely a better a writer. For my day job I've had to write technical documentation on occasion including for a patent application for a project I worked on in addition to regular design work. People always comment about the crappy manuals that come with most CCR's, well take a crack at writing a better one and publish it. I can tell you writing technical documentation is a tedious, boring and mind numbing work. Now I understand why most engineers hate doing it.

Looks like I cut and pasted the wrong link in the last post. It should have been this one. https://en.wikipedia.org/wiki/Intermodulation By the way the link I posted in error is a good company to use with the builtin utility in many routers to keep your dynamic address updated for remote access when you can't get a fix IP address.

It seems whenever there is a failing with a radio's performance it gets blamed on "intermod". https://account.dyn.com/ This seems to be the go to bogyman to explain the problem. I'm not so sure that's the case with the frequency it gets mentioned. There are at least two other factors to consider. The first is image response. Given the general lack of filtering on the front end, which by the way is also a problem with the name brand Japanese radios, might be a more likely cause of poor signal rejection from off channel signals. https://en.wikipedia.org/wiki/Image_response The second is based on the fact many of the CCR's use the ubiquitous RDA1846 radio-on-a-chip design. https://www.w5txr.net/upload/RDA%20%20Microelectronics%20RDA1846.pdf This chip is effectively a software define radio, a cheap one. The max clock frequency of the chip from the datasheet I have max's out at 26MHz. That's way too slow for UHF communications for a normal sampling period to be used, theory says it must be at least twice the highest frequency of interest, so something else must be going on. Normally in DSP applications aliasing is avoided, however it can be used as a type of frequency down conversion method. https://en.wikipedia.org/wiki/Aliasing https://en.wikipedia.org/wiki/Digital_down_converter http://hunteng.co.uk/pdfs/tech/ddctheory.pdf For a typical application example of the chip have a look at a schematic for the UV-5R radio from Baofeng, one of the original CCR's. http://static2.rigreference.com/manuals/baofeng/baofeng-uv5r-circuit-diagram.pdf Given the low clock frequency of the chip, relative to the frequency of interest, several signals, at different frequencies, could end up being down converted and appear to be on the same frequency. Some comments about the Anytone D578UV were made in some other posts. Additional info on that radio can be found here. http://members.optuszoo.com.au/jason.reilly1/578mods.htm About 3/4 of the way down the page is some interesting details about the internal design of the radio, specifically about the IF stage. The comment about the possibility of the third unpopulated IF strip being used in a future model for AM air band RX is nice.

The real question is are the radios being sold in the US? If so what is the FCC ID. With that in hand checking the FCC certification test report would be the definitive answer to radio’s operation. In the case above it looks like somebody did a particularly bad job writhing up the spec’s. Again mistakes in the spec’s doesn’t mean the radio doesn’t exist, or will soon. I ran across a brochure for a Kenwood commercial radio that claims an FCC certification it does not have. Even the company I work for get spec’s wrong. Our sales manager ran a full page color glossy ad on the back cover of a very well known trade magazine years ago with a glaring error that almost got her fired when the company president found out. That happened because nobody from the engineering department reviewed it for accuracy before publishing it.

Try looking here. Windows 2000 likely will work for most of the old CPS versions out there. https://winworldpc.com/library/operating-systems#

Yes I saw the MOQ number. That doesn't imply you can't buy them in single units from a vendor somewhere. The point of the exercise is there are radios available for the 900MHz ISM band. If somebody really wants one they can do the searching for a vendor. Everyone makes errors in their spec's, or the real engineering spec's changed after the original spec's were published. Some just do a better job of making far fewer errors in the published spec's or feature lists. There are other license free radio bands in the US. Some people have been experimenting on 49 MHz to 50 MHz, right below the Ham 6M band. Power is limited to 100 milliwatts. https://www.qsl.net/49mhz/ One other thing to point out is the 900 MHz ISM band is also the Ham 33cm band so you have to share it, and with old cordless phones and the other junk you find on it. https://en.wikipedia.org/wiki/33-centimeter_band https://w6aer.com/getting-started-with-900mhz-ham-band/

There is a far cheaper analog radio to look at also for the 900MHz ISM band. https://www.retevis.com/Retevis-900MHz-License-Free-Two-Way-Radio/

I think you're referring to the ISM frequencies. Motorola sells 50 channel digital radios for this unlicensed service. https://www.motorolasolutions.com/en_us/products/two-way-radios/commercial-business-two-way-radio-systems/on-site-business-radios/dtr-series/dtr700.html#tabproductinfo Here is the brochure for them. They can do 1 watt. https://www.motorolasolutions.com/content/dam/msi/docs/business/_documents/specifications/dtr700_spec_sheet.pdf If they aren't too expensive they could be fun to get a couple to play with.

Yeah, after I went back to read some other posts it made more sense it was a PL tone. My mistake.

Just an additional question was that a GMRS frequency, 462.650/467.650 with a tone of 151.4? It wasn't exactly clear to me when i read it the first time around. The frequency 151.4 looked like a typo for one of the three MURS channels in the 151 MHz range.

I wouldn't say they "squatted", but more like the FCC ignored the illegal operation to the point of no return. In fact you could say the FCC screwed up by commingling FRS and GMRS together by allowing dual service radios in the first place. It's really no different than the old CB radios days when so many people got radios and skipped getting the license. After a while the FCC gave up and made 11M effectively license free. They made their problem go away by making the unlicensed illegal operation now legal. Oh, about a restaurant employee demanding you get off "his channel", well go look for videos and stories about Walmart and MURS. Seems like some of those places think they own those frequencies too. Don't forget that MURS is open to various uses including wireless intercoms, digital data etc. You'll have that to look forward to as well when more people start using it. Oh, when you were on 151.4MHz were you using an FCC certified radio for MURS and what model was it? There aren't that many out there. If the radio was certified before the FCC created MURS, Part 90, it likely would be legal to use when programed with the correct output power and channel bandwidth. I have 5 Kenwood TK-270G-1 5/1 watt 128 channel radios for just that reason plus they work on 2M as well. I started another thread a while back on this form asking how many people use MURS.

Do you have a way to check the line impedance with the cap's and RF chokes installed? At UHF it doesn't take much stray capacitance in a coil winding to end up being a parasitic resonate circuit which can totally bugger up what you're trying to do for example. The best way to check the final setup is to substitute a known good 50 ohm RF dummy load, rated for at least 500 MHz, for the antenna on the load end, then check the match on the feed end. The dummy load should have either a SMA, BNC or "N" type connector on it so the connector doesn't affect the measurement. No electrical component is perfect. Coils have high resistance at RF due to skin effect, stray capacitance between turns. Capacitors also have resistance, dielectric losses and skin effect in the plates, additionally they exhibit inductance due to the finite length of the connections, or in the construction if the capacitor elements are rolled into a cylinder. Then with coax cable you get impedance transformation effects if it's not terminated in the line's characteristic impedance. You can minimize that problem by using multiples of a 1/2 wavelength of cable. Transmission line theory says a 1/2 wavelength of cable will show the same impedance on the feed end as the load end independent of the line's impedance. Remember the wavelength for the cable is the free space wavelength of the signal multiplied by the velocity factor of the cable you're using. For example your typical RG-58 coax has a velocity factor of 0.66 so at 465MHz a 1 wavelength is about 64.3cm in free space and 42.4cm, 16.7 inches, in the coax.

On some of their radios they do in fact have MDC-1200 and state as much in the brochure. See the PDF file below for an example. I have a couple of these radios in my "collection". https://www.kenwood.com/usa/Support/pdf/TK-2360_3360.pdf

Well they can go out and buy the new cheap 2 watt legally FCC certified FRS radios, and yes they can do 2 watts on 1-7 and 15-22 under the new rules, 2017, which became effective starting in 2018, then its back to business as usual. https://en.wikipedia.org/wiki/Family_Radio_Service FRS -> UHF CB radio

Is "licensed" CCR junk on your repeaters a problem too?

Whatever radio you use be sure you can lockout unwanted channels from the radio's front panel. The first cheap Chinese radio I got to screw around with years back was the ubiquitous UV-5R. I quickly found out you can't change the scan options without using the programming software. It's been in the box for the past several years just because of this. I use my radios for scanning very often, and like you I may need to lock out a busy channel at times. One radio I got for digital use, DMR on the Ham bands, was the Anytone D878UV. The hardware seems good enough, but the software is something else. The radio firmware allows scanning channels, and you can organize memory channels, there is a total of 4000 in this radio, you want to scan into scan lists. The part that sucks is when you select a scan list to use the current channel is ALWAYS included even if you NEVER put it in the scan list using the programming software. The current channel happens to be whatever is showing on the display before activating the scan function. And there is noway to delete it either! I can delete any of the other channels in a selected scan list, just not the "current channel". Contacting Anytone I found out this was a deliberate design choice and not a bug. They seem to have zero interest in changing this. I have a big collection of various models of HT's, mostly Kenwood, and NONE do this except for this one radio when using the scanning feature.

Or they setup their own repeater and find repeater jamming isn’t so funny when they’re the ones suffering with the annoying behavior.

I see you're also a Ham so maybe the local club have a few guys that are into Fox Hunting with the required direction finding gear. Repeater jamming is something they take seriously since it does require a significant investment in time, money and equipment to put a repeater in operation. Some of the better equipped guys have Doppler direction finding equipment. The jammer only has to be on the air for a very short time to get a fix. I've seen a few using the system below. http://www.kn2c.us/ Also when you see police cars with those four short antennas in a square pattern they are using a similar system, LoJack, for locating stolen vehicles. The patent application covers the details. Starting at figure 6, sheet 5 of 7, you can see the 4-square antenna setup towards the bottom of the diagram. https://patentimages.storage.googleapis.com/5a/bb/67/410cde79598750/US4908629.pdf https://en.wikipedia.org/wiki/LoJack

Here are some suggestions from somebody who had similar problems. http://www.repeater-builder.com/k7pp/art005.html

You can start looking at the site below for basic information on how to build a system. http://www.repeater-builder.com/rbtip/index.html

That won't go over so well is absolutely right. The Ham bands are ALWAYS being looked at by various groups to use. Hams already lost part of the 1.25 meter band due to UPS wanting it. At the site below look under the section heading "U.S. reallocation" for a brief history lesson. https://en.wikipedia.org/wiki/1.25-meter_band Hams have to constantly watch their bands for illegal CB'ers on the 12M and 10M bands, local TV stations on the old analog channel 2 made 6M almost unusable in those areas, fishing boats - high power Chinese cordless phones among other junk operating illegally on 2M, and on 70cm there is a low power commercial allocation at 433MHz, right in the middle of the band. Hams also had to deal with some companies using parts of 70cm for police drones etc. not to mention the band is shared with the US military. The later was an issue with the military Pave-Paws radars. http://www.arrl.org/news/air-force-pledges-continued-cooperation-with-radio-amateurs-during-cape-cod-radar-upgrades

Oh, one more thing. Look at the ID tag on the back of the radio. It has the manufacture’s model number on it. I’ve seen too many eBay sellers get the radio description wrong. The only way to not get stuck buying a model that won’t work on GMRS is to check that ID tag. Most sellers show it in one of the photos of the radio. If not contact the seller and ask for the exact model number that appears on the ID tag or request they add a photo showing it.

Be very careful which one you get. There are several different models of the TK-880 which covers different sections of the UHF band. The one you want is a Type-1. Look at the last page under specifications. The Type-1 is the only model that covers the GMRS frequencies and is Part 95 certified. https://criticalradio.com/Kenwood%20Spec%20Sheets/Data%20Sheets/TK780h_880h_ver2_k.pdf

Maybe at some point they will issue a rule change which specifically requires it and end the debate. 8-) Until then there is some wiggle room. Even in the Ham world standard offsets are not always used. The following is for a 2M machine out where a Ham buddy lives. https://www.repeaterbook.com/repeaters/details.php?state_id=26&ID=10560 The "standard" for the 2M band is 0.6MHz. But not in the above case and it is a coordinated repeater, apparently, as you'll notice towards the bottom of the page. https://wiki.radioreference.com/index.php/Offset In the early days Hams used converted commercial VHF and UHF repeaters. On UHF the 5 MHz offset, I think, was mostly to do with the technical limitations on the cavity filters used. Here is some interesting reading on another forum. https://forums.qrz.com/index.php?threads/history-of-vhf-repeater-splits.26321/