Lscott

Well you summed it up right. The mystery is what effect is more dominate in real usage conditions, free space path loss, signal absorption etc. The impressive part is on UHF one needs significantly more power on UHF to generate the same signal strength at the receiver compared to VHF. That’s assuming you keep other factors about the same. In another topic I started I had asked just how much activity do people hear on the license free MURS channels, which are on VHF from around 151 to 154 MHZ. Given the issue of path loss and MURS radios limit to 2 watts I’ll guess it could outperform a much higher power UHF radio.

"www.bccdc.ca/resource-gallery/Documents/Guidelines and Forms/Guidelines and Manuals/EH/EH/Section2Final06062013.pdf"

That would be an interesting test. I often read where VHF tends to reach further because at the lower frequency the RF is absorbed less by tree leaves etc. I'm not so sure that's the main reason. There is another one that could explain it more called "free space path loss" which has nothing to do with signal absorption or blockages. https://en.wikipedia.org/wiki/Free-space_path_loss http://www.sis.pitt.edu/prashk/inf1072/Fall16/lec5.pdf What it comes down to is the signal strength is expressed in "volts per meter" and is independent of frequency but related to transmitter power. "http://www.bccdc.ca/resource-gallery/Documents/Guidelines and Forms/Guidelines and Manuals/EH/EH/Section2Final06062013.pdf" However since one has to use a resonate antenna, or nearly so, the antenna on UHF is roughly 1/3 the "length" of the same type at VHF and thus intercepts just 1/3 of the signal expressed in "volts per meter". Thus the received signal is 1/3 the amplitude, voltage wise, coming out of the antenna. From a power stand point the received signal "power" is proportional to the square of the voltage thus the "power" at UHF would be about 1/9 that at VHF, or in db's, 10*log(1/9), its 9.54db lower. https://www.allaboutcircuits.com/tools/free-space-path-loss-calculator/ Using the above calculator the path loss is 75.962db with the following data input: Distance: 1 KM Frequency: 150 MHz Transmitter Gain: 0 Receiver Gain: 0 And you get a path loss of 85.504db with the following data input: Distance: 1 KM Frequency: 450 MHz Transmitter Gain: 0 Receiver Gain: 0 The difference is. 9.542db lower as expected on UHF compared to VHF for the two frequencies used. The higher the calculated number is in db the higher the loss.

Thanks for the tip. I’ll have to try it the next time. Having the links get messed up sort of ruins things.

A good place to start is some answers to the following questions. 1. Used to communicate primary with just handheld radios? 2. Used to communicate primary with just base radios? 3. Used to communicate with base and handheld radios? 4. Is repeater access required and how far away? 5. Do you need multi-band access, like for Ham and GMRS? 6. What is the terrain like where you plan to operate most of the time? Flat, low rolling hills, mountains, lots of tall building etc. 7. How tall of an antenna can you tolerate? Like getting in to a garage, parking deck, drive through window heights etc. 8. Is a magnet mount needed or are drilling mounting holes in your vehicle OK? 9. How much money do you want to spend? There are a lot of antennas out there from a few inches tall to several feet costing as little as $20 and way up from there. Once you have an idea of what the requirements are people here have some good solid recommendations. Picking an antenna is like going to a buffet. There is too much to choose from.

Well the only way to get this to work is copy the whole URL into the address window up to the ".pdf" part. Then substitute the following for the "...ents" part. Then it should work for the long one. resource-gallery/Documents For the shorter one copy the whole URL into the address window up to the ".pdf" part. Then substitute the following for the "....ications" part. Then it should work for the short one. Don't miss the leading period. .za/publications

Dang!! This form's software keeps screwing with the URL's, they won't post right. They cut and past OK but when I post the message they get trashed. I'll try one more time. "https://www.google.com/search?q=field+strength+versus+power&client=firefox-b-1-e&ei=WsY6X4GYI8-PtAaf9JOAAw&start=10&sa=N" "http://www.bccdc.ca/resource-gallery/Documents/Guidelines and Forms/Guidelines and Manuals/EH/EH/Section2Final06062013.pdf"

Hummm. Firefox doesn't cut and past links very well at times. These should work I hope http://www.bccdc.ca/resource-gallery/Documents/Guidelines and Forms/Guidelines and Manuals/EH/EH/Section2Final06062013.pdf http://www.parc.org.za/publications/=Field strength vs radiated power.pdf

On closer inspection the ERP is based on the field strength of the signal. That's the point people seem to miss. A gain antenna increases the "E-Field", Volts per meter. When the tests are performed the location of the field strength meter has to be specified. When doing antenna testing on an antenna test range the "E-Field" is measured at various points around the antenna. There are relationships that you can use to calculate power (ERP) based on the "E-Field" strength. https://en.wikipedia.org/wiki/Surface_power_density The references below gives a bit more info on how the "E-Field" in Volts/Meter works out to power. Note that a number for the antenna gain is part of the calculations. http://www.bccdc.ca/resource-gallery/Documents/Guidelines and Forms/Guidelines and Manuals/EH/EH/Section2Final06062013.pdf http://www.parc.org.za/publications/=Field strength vs radiated power.pdf https://en.wikipedia.org/wiki/Effective_radiated_power There are two ways of looking at this EIRP, effective isotropic radiated power, and ERP, effective radiated power. The two are not the same. For EIRP it's assume the power is spread uniformly over a sphere, which will only happen using a "theatrical" isotropic antenna. The other, ERP, acknowledges that real antennas have some directional properties, thus a higher "E-Field" in some directions verses others. When the "E-Field" is measured then power calculated you will most likely end up with a power higher than what you see at the transmitter's output. That's the antenna's gain.

So by that definition even a high gain Yagi really wouldn't be violating the rules. There is only one driven element (antenna) everything else is either a director or reflector element.

Logically I can't see how limiting the output power will prevent exceeding the FCC's ERP limit. Since they have no idea what antenna is going to be attached it's impossible to set a power level that will not violate the rules, unless it's set at zero. You don't even need to attach a gain antenna. A few people have built a corner reflector and just stuck the radio at the right point which results in a higher ERP than what you get out of the radio, even with a fixed mount antenna. The radio is place at the position where the dipole element would go. Then ran a external headset with a conveniently long wire to the accessory input jack on the radio. https://www.qsl.net/ve3rgw/corner.html While not exactly portable its been done for point to point communications.

The usual rubber duck antennas, stubby antennas, have a negative gain. To get the 0.5 watt ERP the radio would have to produce more than 0.5 watts. Clearly the market for the radio is GMRS. The low power narrow band channels are an afterthought looking at what components were used. The point is anyone who is considering this radio with the idea of using it to talk to FRS radios, or have a real need too, will likely be disappointed. If the radio does what you want that's what counts. At least people know a bit more about the radio's likely performance and can make a better informed choice. That was the goal here.

One point not mentioned much are the gain figures work both ways. That is on transmission and reception. It pays to optimize the losses. A 5 watt radio likely is about as sensitive as a 40 to 50 watt one. You can run into cases where spending more money on a higher power radio to make up for the power loss but it does nothing to increase the receive signal strength. If you can’t hear the other station it doesn’t matter how much power you run. This could be the difference between using a 5 watt handheld with a roof mount antenna, or spending a lot more money on a high power mobile radio. I think enough information is here where choices can be made that fit budget and physical installation requirements. There shouldn’t be any really big surprises how the final system will perform.

I forgot to include a datasheet for regular RG-58U cable. https://catalog.belden.com/techdata/EN/9201_techdata.pdf As you can see the loss is almost double at 8.4db at 400MHz. The losses go up as the frequency increases. One table I have shows it at 10.6db per 100 feet at 450MHz. With this kind of loss one would do better sending smoke signals.

You should consider using a different antenna. The MXTA25 is listed as a 5/8 wave design. https://midlandusa.com/product/micromobile-mxta25-3db-gain-ghost-antenna/ From modeling this antenna has a net gain LOSS over a simple 1/4 wave, or 1/2 wave dipole antenna. Where you want the signal strength at the maximum is along the ground and not shooting up in the air at an angle. https://www.w8ji.com/VHF%20mobile%20vertical.htm The low loss cable you mentioned I found a datasheet for it from Belden Cable. The loss spec is 4.9db per 100 feet compared to LMR-400, 2.7db per 100 feet, at the same frequency of 450MHz which is very close to the GMRS frequencies. https://catalog.belden.com/techdata/EN/7808WB_techdata.pdf https://www.timesmicrowave.com/DataSheets/CableProducts/LMR-400.pdf For the MXT400 radio there is an FCC test report you might find of interest. On page 7 it has the high and low power measurements. https://fccid.io/MMAMXT400/Test-Report/Test-Report-3120260.html

A small typo I made. The gain should have been 6.85db. The rest of the numbers should be correct.

I’ll leave that as an exercise for the student to figure out the system gain as they instructor used to say.

Ah no. When you see gain spec of 9dbi that’s a gain over an isotropic antenna that doesn’t exist. It’s for reference only. What you really want is a gain spec over a dipole, or a simple quarter wave, which is the smallest antenna that can be physically built. The difference in gain between the two is 2.15db. So the real practical gain is now 6.15db. But wait, we’er not done yet! Now you have to figure in your coax loss. For a typical run of 75 feet of LMR400 at 450MHz, close enough to the GMRS band at 462 to 467, the loss is another 2.03db. So your real gain is around 4.82db, which is a factor of 3.03 increase. For 5 watts in it looks like you have an effective radiated power of 15.17 watts.

Boxcar has a good suggestion. A few more points about MURS. You can use external antennas and the VHF signal seems to propagate further through trees etc. And finally you don't need a license to use MURS radios, but they must be FCC certified. GMRS radios everyone needs to have their own license unless they are a qualified family member of someone who is licensed. One thing I have noticed about operating FRS radios inside vehicles, the range is poor, around 1/2 mile is typical. If you use GMRS radios you REALLY need to use a roof mounted antenna, even with a handheld radio. Same point about MURS radios too.

You should be aware the Midland radios as far as I know are all narrow band FM. Look at page 4 "Type of Emission". https://fccid.io/MMAMXT400/Test-Report/Test-Report-3120260.html The "10K2FE3" is the FCC's technical definition for narrow band FM. For GMRS it typically should be "16K0FE3". Unless the handheld radios can be programmed for narrow band FM you might want to think about getting them. The issue with miss matched radio bandwidths has to do with the audio at least. A narrow band radio receiving a normal FM bandwidth signal will have a very loud audio, and may distort it. Going the other way a normal FM bandwidth radio receiving a narrow band FM signal will have low audio level. Trying to talk to more than one station with miss matched bandwidths everyone will be playing with the volume control and that gets old real quick. As a guide a doubling of power output may get you at most a theoretical range increase of around 40 percent. There are lots of other factors at the GMRS frequencies that will conspire to reduce this. Many have reported not seeing much of a change at all. You can use a radio from any manufacture to communicate with another manufacturer's as long as they are programed with the same frequencies, and tones if used, with the caveat above about the bandwidth.

Thanks. I missed that. I guess that's what I get for not looking a bit more carefully and just doing a quick scan through the datasheet and dashing off a post. I'm happy to see somebody is interested in the topic enough to look. 8-) So that now brings up the flip side question. Is the bandwidth now too wide? If it's 24KHz then it's way too wide for narrow band FM at 11 KHz. Now it's the reverse of what I wrote in error. Now the normal FM mode is likely OK but not the narrow band mode. Oh well. The FM deviation set for 2.5KHz and the audio gain increased, necessary in narrow band mode using the wide band filter to compensate, will work but the selectivity would suck. Since GMRS is 5KHz deviation narrow band performance likely isn't a big concern. One thing I didn't point out is the power output on the FRS only channels is 0.283 watts as shown in the grant. The FCC allows up to 0.5 watts so this radio won't even do the max allowed output power for those channels. If a potential user is looking at the radio with the idea they may need the narrow band selectivity and the max allowed power on channels 8 to 14, because of a need to communicate with FRS users, may want to consider another radio. In any case looking at the electrical design reveals an aspect of the radio's likely real world performance that isn't mentioned by the manufacture. At least for the GMRS specific version of the radio.

For the more technically inclined people you can investigate a design by looking at either the schematic, in this case it's not available on the FCC website, or the internal photos, which is better than nothing. https://fccid.io/WVTWOUXUN16/Internal-Photos/Int-Photos-4695706.pdf Looking at the PCB photos you will notice a large white rectangular part with "C50F" stamped on it. I've seen these before. They are commonly used ceramic filters. In this case doing a bit of searching you will find a data sheet for it here. http://www.quartz1.com/downloads/Tecdoc/filtr_ceramic/LTWC450F.pdf Take a look at page 4 of the datasheet figure 1. You will see the manufacture's marking looks like the one in the photos. As suspected it's a simple 455KHz IF, intermediate frequency, ceramic filter typically used in a Superheterodyne receiver designs. This would not be something expected in a typical "radio-on-a-chip" design. One thing to note in the spec sheet is the 50dB bandwidth spec of just 12KHz. Considering GMRS has a bandwidth of around 16KHz the filter is a bit narrow. However for narrow band, think FRS, the bandwidth is only 11 KHz it would be a bit too wide. I suspect the designers picked this part as a compromise where they tried to get away with using just one filter in place of the two that should have been used to save money. I think some of the commercial radio designs use two different filters for the two bandwidths. One reason why they tend to work better and cost more. And right next to it is a chip "AA32416" which appears to be the FM detector chip which would make sense. https://www.digchip.com/datasheets/parts/datasheet/849/AA32416-pdf.php Radio internal photos. https://fccid.io/WVTWOUXUN16/Internal-Photos/Int-Photos-4695706.pdf And for those who wonder what a Superheterodyne receiver is there is a nice history and write up here. https://en.wikipedia.org/wiki/Superheterodyne_receiver Looking up the numbers for the other chips, assuming they are not proprietary part numbers for the end user, one might gleam some other interesting details about the radio and it's likely performance.

You might want to try using a different HT. It’s shocking how deaf some HT radio’s receivers are. Particularly the cheap ones.

You can try plugging numbers in to the calculator at the site below. http://www.hamuniverse.com/lineofsightcalculator.html

That could be the case. You can see the FCC data on the radio at this site below: https://fccid.io/WVTWOUXUN16 The FCC certification grant is there. More of interest you can find internal photos of the radio. After looking I didn’t see the typical radio-a-chip device, which seems to be the favorite used in the cheap Chinese radios, on the circuit boards. I didn’t try to look up the chip numbers so the manufacturer could still be using a different one, or a customized version.