Lscott
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Posts posted by Lscott
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Nice radio but it appears not to be certified for FCC Part 95 GMRS use.
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OK. You’ve got me thinking. I started going through linked items and thought to myself. “What the heck. You just got off work and its time to let the mind rest.”
Anyhow, On a related note I found this little diddy that relates to the topic. It as part of this stackoverflow post:
https://ham.stackexchange.com/questions/3692/is-free-space-path-loss-dependent-on-frequency
The study of RF propagation in air is new for me so I am going to need some time to digest. It sounds like this is something you are really passionate about. What seems to make perfect sense to me is that if you radiate a given amount of power from a point source in space that the intensity of that power at some given point a given distance away would be the same, regardless of frequency. However, when considering the physical size of the capture device, larger wavelength devices feature larger capture areas thus the amount of power impacting the antenna would be greater. Using microwave dishes as an example, the dish is quite large compared to the wavelength. The dish provides a large area to capture the radiate signal to reflect, redirect and focus the energy onto a small wavelength antenna. So, because the antenna is physically small, some means is needed to capture more of the available energy.
Now that I am entering the ham world more seriously I imagine I studying more things like this.
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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.
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"www.bccdc.ca/resource-gallery/Documents/Guidelines and Forms/Guidelines and Manuals/EH/EH/Section2Final06062013.pdf"
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I am also look forward to winter around here, not because I like cold, but because the leaves will be off the trees and I am really curious to see how range improves in the absence of the leaves.
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.
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I have experienced the URL pasting issues in the past too. When I do have such an issue I past the URL in text editor, add quotes around it and then just pasted the quoted text. Quoted text often times prevents the automatic hyper linking. A pain? Yes! But sometimes necessary.
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Thanks for the tip. I’ll have to try it the next time. Having the links get messed up sort of ruins things.
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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.
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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
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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.
"http://www.bccdc.ca/resource-gallery/Documents/Guidelines and Forms/Guidelines and Manuals/EH/EH/Section2Final06062013.pdf"
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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
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Here is FCC Definition of ERP.
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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.
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Not to be (too) picky. But, I am pretty sure ERP is measured at the antenna and would therefore not include any external reflectors, etc.
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.
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Lscott, I agree with you on the rubber duck having negative gain. However, I still think that because that antenna can be replaced with one that has substantially higher gain then it becomes possible for ERP limits to be exceeded. And since for channels 8-14 the spec is max ERP, that might be why TX power is set so low.
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.
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I own a couple of the KG-805G and currently considering getting more. My take on the .283 watts is that it has to do with the FCC requirement for no more that .5w ERP, not .5w transmitter output power. If the radio did output .5w, then putting a good external antenna on the radio would cause it to the exceed the max ERP rating for channels 8-14. FCC clearly wants these channels left for close-in communications only.
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.
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No disagreement. Only a reminder to readers of a point @Jones made earlier. There is a place for thin cables, particularly in a mobile environment. Also, in a case where the radio is at a window and the antenna line goes out the window and a few feet up to an antenna.
Noted in these forums before, but always important to keep in mind: Perfect is the enemy of good. If someone deployed 50 feet of RG58 to their antenna and can reach all the repeaters and mobiles they want, then that is probably a good time to stop.
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.
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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.
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I just received my MXT400VP3 Micromobile Bundle with a MXTA25 3dB Gain Antenna and plan on using it as a base unit, mounting the antenna approximately 25 feet up on a Tri-pole TV antenna tower.
I know there is an MXTA11, and an MXTA26 6dB Gain Antenna available for use with the MXT400.
Is there that much difference Performance wise, that it would be advantageous for me to upgrade my antenna from the 3dB gain to the 6dB gain antenna? By performance I'm speaking of increased range and clarity of signal.
If so, which would be the best to choose? The MXTA11, or the MXTA26 ?
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
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So the real practical gain is now 6.15db.
A small typo I made. The gain should have been 6.85db. The rest of the numbers should be correct.
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It's worse than that, he said he will be using Rg58u.
I’ll leave that as an exercise for the student to figure out the system gain as they instructor used to say.
- DeoVindice and PRadio
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BUT... For channels 1-7 the FCC limits the power to 5W ERP (Effective Radiated Power). So if we run at 5W and have an antenna gain of 9dBi, the ERP will be 30W!
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.
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The other option is MURS, a VHF service limited to 2W but because its lower frequency it will have a marginally better range than the UHF GMRS/FRS units.
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.
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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.
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Notice the plus/minus symbol right in front of that 12KHz rating... that means the bandwidth is +12 and - 12 from the center zero point of 450KHz. That's 24KHz total 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.
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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.
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All my tests have involved use of a pair of Wouxun KG-805g 5 watt HT.
You might want to try using a different HT. It’s shocking how deaf some HT radio’s receivers are. Particularly the cheap ones.
My Range Experience, Looking for Input
in General Discussion
Posted
Yeah, until the local PD shows up because you look "suspicious" and won't believe you're just doing a radio range check. Even a few Hams get hassled by the local PD for having a "scanner" in a vehicle and or hearing police radio dispatch calls coming from their VHF/UHF mobile. The recommendation is keeping a copy of the local/state law showing the exception for Hams handy along with your license just in case in the glove box.