WRUW493

This is why RF engineers use dBm instead of watts. An additional 5 watts starting from 5 watts is 3dB. But an additional 5 watts starting from 50 watts is 0.4dB. The former is noticeable, the later is not. The traditional "S-unit" is chosen to be a difference of 6dB, which is usually considered the increase in power necessary to be a meaningful increase. Or at least enough of an increase that the RX signal strength is enough stronger to be worth the extra (insert here: antenna gain, TX power increase, etc). I can't find it now, but the FM receiver parameter called "rise" is a measure of how high the signal is above the RX noise floor has to be to achieve a desired signal to noise ratio, ie SINAD. This is not a linear curve as in an extra 1dB in "rise" does not equate to an extra 1dB in SINAD. So here again, RF engineers like the dBm power metric as it lends itself to speaking of a change in "rise" more easily. Hope this helps.

Yes, I agree. Furthermore, some of the fiberglass radome verticals that are supposed to be co-linear arrays are not done well, or are using a 70cm ham band design, which don't give a good radiation pattern at GMRS. Good VSWR can be achieved with a terrible radiation angle, ie way above the horizon, if not careful. I've seen similar antennas on eBay and Amazon that claim to be for GMRS yet even the photos show they are dual band 2m/70cm labeled. OK, I've pretty much talked myself out of this one already. But any real world experience would be nice to know. thanks.

Does anyone have any experience with the "FMUSER" GR100 base antenna? It's on sale for Amazon Prime days now and according to the advertised specs it's pretty good. But is also lower priced than a similar Tram which makes me suspicious. Thanks for any thoughts on this antenna.

Sounds good LeoG. I fear that Retevis made a mistake when they upped the TX power. They either didn't have enough isolation in the duplexer, or choose to allow more insertion loss to get a high isolation. Either way is makes for poorer RX sens. I hope I'd dead wrong though and it's good. According to the spec sheet, the 97S is 0.20uV, which comes out to -121dBm. Many years ago when I worked on a project that had a full duplex radio I modified it (in a kludgy way) with a 1dB NF preamp and custom bandpass filter to protect the low NF preamp. The resultant RX sensitivity was about -123dBm, which is as low as you can go in the real world on a 25KHz channel BW. So the -121 of the RT97S is pretty darn good for a "mere" consumer grade repeater.

True. Well OK, but the RX sensitivity at the antenna connector is what matters. Do we know anything about how good or not that is? ps: yes IM, Adjacent Channel etc matter in some installations too

I wonder how good the RX sensitivity is on the 97L. I read somewhere that it's spec'd at 0.35uV, compared to 0.20uV for the 97S. Any one know if this is correct? ps: I hope not as 0.35uV is not very good for a repeater. It would have a hard time hearing the HT's when far out but the 20+W TX would get to them well. thanks for any info.

Regarding the Red's repeater controller: first, nice job of testing it out. Seems like it has some nice features. Question: As I see in reviews of the Surecom, there is a practical problem where when recording audio (on the incoming transmission) if there is a pause in the speech the controller will think that is the end of the transmission and then start the repeat playback. When in fact the incoming transmission is not yet finished and thus only part of the statement will get repeated. How did you find that the Red's behaves under such a condition? thanks for any info.

https://www.youtube.com/watch?v=G44BtmgYqhw Looks to be a rather simple mod. Was hoping to find something even easier Yea, this turns it into a TYT radio, as you say and in the video.

I have a couple of GM-30's that my wife and I use in the Idaho outback for safety comms. I'd like to open it up for 70cm (440) ham band TX. I have put a UHF only antenna on it for improved performance over the OEM dual band antenna, thus I don't really need 2m / VHF TX. I saw a youtube on how to program it with Baofeng (?) firmware opening it all the way up. But I'm hoping for just some "push these buttons while turning it on" action to open up the 70cm band TX. Any suggestions? thanks in advance.

Yes they possibly are. Although it's hard to tell. The current listings for example show them as old stock, built in 2012 with genuine (looking) Kenwood markings. But to be fair it's a crap shoot as to what it is. Best I can tell the original Kenwoods were also made in China, so maybe there is a difference maybe not. I've just ordered the 2m version (281) and will access that when I get it before I make any definitive claims. There does seem to be two versions on the UHF model, one 420-440 the other 440-480. So one at least has to make sure to get the proper one. I've worked in the 2 way radio / cell phone business some years ago and found that many of these types of products are made in China even by the reputable US or European manufacturers. Lots of times the China factory will keep making the same model after the US manufacturer gives up selling them. Sometimes the China factory then goes and cost reduces it, thus compromising performance/quality/features. Sometimes they just keep it as is. The one thing I can say for sure is that the Chinese engineers are smart, and frequently very smart when it comes to design for manufacturing aspects. It's a crap shoot for each type and model one looks at. The only thing I do observe is that the Kenwood 281/481 models seem to have been flops in the US ham market. Possibly because they were single band in a time when dual band was all the rage, and rather lame single band at that with only 100 memories.

Well, not quite correct on the V vs H polarization. Yes it's true that either is fine for line of sight propagation, it's not so true for terrestrial scattered propagation. The difference some times is minor, but not always minor. The main difference is that many RF absorbing objects out in the real world are horizontal. Take tree branches, leaves and needles for example. Additionally the ground itself, being mostly a horizontal air to ground interface, can have strange reflections that can cause weak signals. So the generally preferred polarization is vertical. And yes, you should try to match the polarization on both ends. Clearly vertical is easier to implement on a mobile/car/UTV etc. Hope this helps.

I'm considering a Kenwood TM-481a, available on eBay. It's a model year 2012 for new old stock, so a bit on the old side, but the 281/481 are ham versions of Kenwood's commercial line. In the case of the 481 it's wide band and will do GMRS. Nothing fancy, but reliable, not large, good TX and fantastic RX. I find the Midlands are high priced and under performance, sadly.

Right on the higher vs lower in the above post. Further, at heights of "merely" 40-50ft in a wooded area, you will do better with a lower gain that is higher elevation than a higher gain lower elevation. Getting above the trees is paramount if you can do so. Feedline losses are important too as previously mentioned. Another thought, IMHO, never, never, never, did I say never, use 9913 coax unless you are certain how to seal it from the weather. Any amount of water inside that stupid coax will kill the performance in a heart beat. Hope this helps.

Keep in mind that the electrical distance between the base of the antenna and the "ground plane" of the vehicle is important. Two inches at these frequencies it too long and will mess with VSWR as well as radiation angles. A trunk lip mount for example has a rather short distance where some of these other brackets that have 1, 2 and more inches of distance is way too much. also, with the only exception being an end fed half wave, the horizontal radiation pattern will essentially mimic the ground plane below. This is usually the hard part, getting some ground in all directions. For both of the above reasons is why an NMO mount in the center of roof/trunk is THE BEST. A mag mount in those locations is good too but cable routing is a problem. And yes, I too have mounted antennas on the edge of the vehicle sacrificing horizontal pattern for convenience or cosmetics. One last thought: at these frequencies there is very little difference in performance between a roof mounted vs trunk mounted location. The RF will just go through the passenger compartment with little distortion. And the added height above ground of the roof mount will make no noticeable difference too. That is why you see most cop car sedans with the antennas on the trunk. Hope this helps. And yea, I know you don't have a "trunk" on the Bronco, I used that example in the generic sense.

I'll give my take on it from an RF antenna engineer perspective... In a few cases that I've seen the coil serves to make what should have been a longer antenna shorter. The classic example is a helical on a handheld, say for the 2m ham band, that should have been 1/4 wave, ie about 19 inches. The coil'ed up whip turned into a "rubber ducky" is electrically 1/4 wavelength long courtesy of the "mutual inductance" of the adjacent winds of the coil. Such an antenna is not 50 ohms but can be close-enough if the diameter/coil pitch and over all length are tweaked a bit. (did you know that a straight 1/4 wave whip is not really 50 ohms either, we shorten it a bit to bring it closer to 50 ohms). As for a coil in the center or there's abouts, of the mobile antenna: That can be to shorten it similar to the above. But most likely it's to give the proper phase relationship (read RF time delay) between the upper and lower straight parts of the whip. This is actually rather simple to think about if one comes to a curious understanding of how antennas work. Let me give that a short try.... The above diagram was pasted in from the web. The red line shows the magnitude (distance from the black line) and phase (which side of the black line) the RF current is on the various wire lengths (in fractions of 1 wavelength). Note that the top end is always zero magnitude. Has to be, at the end of the wire there is no where for the RF current to go, so one can literally start at the top of the whip at zero current and draw the above curves from the top down. Let's look at the 1/2 wave example. If I could place two of these antennas, one on top of the other, then view this from a long ways away at the horizon to the side (where we want the radiation to go), we would see two RF current sources with the same phase and magnitude adding up in my desired direction. We call this "gain". ...gain being more signal in that direction compared to a dipole. But look at the 3/4 wave example above and note that the bottom 1/4 wave has the phase opposite of the top 1/2 wave (assume that the bottom extends below the horizontal line). Such an antenna does NOT have these two RF current sources adding towards the horizon because they are out of phase with each other. What to do??? The answer is simple: put a phase delay in between the bottom and top 1/2 wave sections so that they are in phase with each other. What could that be??? a coil would do this just fine. It would be good to make this coil physically rather small so that it does not radiate well itself, and of course would have to have enough inductance to perform a 180 degree phase shift in order to bring the top and bottom antenna halfs back to "in phase". This is just one example, but should give you the understanding that it is possible to implement this in a variety of ways with different lengths of top and bottom parts, each requiring different coils. Two 1/2 waves "co-linear" is classic and works well. But the coil has to be 180 degree phase shift and that's a lot of coil. More commonly they use say 5/8 wave over 1/4 wave which requires a smaller more practical coil, but does then suffer some undesired upwards radiation due to that annoying extra 1/8 wave part of the 5/8 wave section. Now we enter the world of compromises. Mobile antennas are full of compromises it turns out. The best antennas are the co-linear base station antennas that are housed within a protective casing (usually a fiberglass pipe) where they can implement the above idea of stacked 1/2 wave sections several times over and the fiberglass pipe supplies the mechanical strength. Some of those can reach 6dB and more in gain towards the horizon, something a mobile antenna can only dream of. I sure hope that makes sense and helps.

Thanks for the internal photos Lscott. Wow, look at that delightful full sized metal shield. No wonder these radios radiate well, they have a huge "ground plane" for the whip. I stand by my statement that if one puts a true and efficient (read low loss, not steel but rather copper) whip on this radio it will radiate very well indeed.

Thanks for the input. This looks interesting. It seems that Menus 16 and 16 have only On or Off choices. I'll play with them to see what they do. I now have a second RA25 on the way which will go in our UTV. So I'll have two identical radios to see how they behave with these settings talking to each other.

yes, agree that the whip needs to be away from other metal, and large plastic etc, objects. When I asked the question I was thinking more of the antenna mount itself. In such case the roll bars for example would serve as the ground plane. I installed an NMO mount on the roll bar of a Polaris General. I custom made a L shaped bracket with mounting holes that aligned up with 2 of the 4 large bolts that connect the center roll bar to the "B" pillar. In the case of Polaris they bolt the roll bar sections together so this was easy to pull out 2 bolts, and install the L bracket under the bolt heads. I found 2 nice star lock washers to go between the L bracket and the roll bar. I hope that the star washer sharp points penetrate the paint on the roll bar and give a good RF connection between the two. The good news is that the roll bars then give a decent ground in the forward, side-ward and rear directions. But the other side direction has no ground therefore I'm sure the radiation pattern is somewhat distorted. But being that it's at the B pillar location hopefully the vertical B pillar offers some symmetry...or so I hope

Interesting point on the human body. But I suspect that human tissue is quite lossy at these freqs. I know for sure that it is lossy at 800MHz and also at 150MHz, so most likely also at 460MHz. Way back in the stone age at Motorola, with one way pagers, remember those....there was a model called the PageBoy II. It had a short hairpin antenna around the outside of the case that when belt worn would couple the magnetic field to the RF currents running along the length of the body. But it should be noted that although this was the "best" position on the human body, that this was still lossy compared to free space for example. It was assumed that the pager would mostly be worn on the body somewhere, and on the belt near the center of a 6ft tall person was typical, and at 150MHz a 6ft person is around 1/2 wavelength tall. Thus near the center of this 1/2 wave lossy body is where the current is maximum, thus the PageBoy II antenna would couple to that. Just a tad of history for you history folks

I'd be interested to see what sensitivity your Mot radio shows. Which model is it anyways?

I see in the Retevis RA25 features list on Amazon that it has the ability to adjust the mic gain. But I don't find it anywhere in the settings or in the manual or in the Retevis programming software. Has anyone figured out how to set this? (I have UTV and want to lower the mic gain to not pickup so much engine noise). Thanks in advance.

This looks good WSAK961. Yeah, in hilly terrain the lower angle of radiation can work against you. I'd say it's due to not radiating upwards to the hill top where the scatterer is to get over the hill. The shorter than 1/4 helical has a near horizon radiation pattern but also is very broad in the Z plane. As the helical gets shorter they eventually turn into nearly spherical in pattern (not polarization that's different). Agree that QC is an issue but I can also say a near 1/4 wave is rather broadband do is tolerant of dimensions. But build QC is a different thing. Nice that you got the wife to tolerate any antenna.

How tall is it? 1/4 wave at GMRS is 6 inches. It would be rather easy to make a 3 inch tall one turn helical antenna that is only a dB or so worse than a straight 1/4 wave which would be impossible for us to observe with just real world testing. Nice review!

That is correct on A weighting. I'm not familiar with Z weighting though. Nice to hear that someone has good test equipment available. Would you by any chance have some mobiles or handhelds available to perform some comparison sensitivity testing on? There is pretty widely available TX power testing capability but RX is much less common.

Direct conversion receivers suffer from rather poor "image", "spurious" and "blocking". All are measures of how well the receiver rejects off-channel signals. SuperHet receivers suffer from rather narrow RF tuning range with good sensitivity. If you want broadband coverage for good Ham band, GMRS, Public Service and NOAA reception, then go Direct conversion. If you want the best you can get (assuming it's well designed) for GMRS only, then go SuperHet. IMHO.