WRUW493

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.

As an RF engineer who has designed receivers for over 10 years, I can say that SINAD is not a receiver metric. SINAD (Signal In Noise and Distortion) is a measurement parameter to which receiver performance is measured. dBm (dB-milliwatt) is far and away the most common receiver sensitivity metric in the RF design world. Sure FM broadcast receivers are measured in dBf, (dB FemtoWatt) but that is not typical elsewhere. Actually, in the receiver world, Noise Figure (F) is the best metric as that speaks to the RF front end without impacts from the audio circuitry, see below. To the person who posted dBm sensitivity measurements: Surely you used some proper RF service monitor to perform this measurement. May I ask what "weighting" you used on the audio? A or C weighted? it does make a subtle difference... Hope this helps.

Slight clarification: Agree with the 5/8 wave needing a good ground plane. And yes strictly speaking the 1/2 wave end fed does not need a ground plane at all. But....what does need a "counterpoise" frequently is the matching from 50 ohms to high impedance in order to end feed the 1/2 wave. If that matching is done with discrete components (caps and inductors) then no counterpoise is needed. If on the other hand the matching is done via a physical structure that radiates, ie: helical 1/4 wave is common and easy, then THAT needs some counterpoise to do-it's-thing. Probably does not have to be large, but should not be zero. Not sure why you would rule out roof pillars though???

Same as above, plus: keep in mind that aluminum is non-ferrous, meaning that is actually repels magnetic fields. Sure it's thin, but there might be a risk of deteriorating the magnet if left on the aluminum long term. Consider a large steel washer below the magnet to "capture" the magnetic fields, then place that combo on the aluminum foil.

I've thought of a way where a longer antenna on the talkie can be an improvement over both the OEM antenna and a 1/4 wave whip proposal. Let's think about this: Many OEM antennas are dual band, for GMRS and also VHF for RX and/or TX. This means that the OEM has to compromise somehow to get it to work on VHF in such a short physical length which inevitably introduces loss at GMRS. Further, as we know, many talkies are not long enough to come close to 1/4 wavelength at GMRS. Thus even with a 1/4 wave whip the efficiency and radiation patterns will be impaired. So how to improve on these situations?? It is true that a good conductor 1/2 wavelength whip will be efficient and have a good donut radiation pattern. That would be about 12.1 inches for GMRS (less than simple 1/2 wave due to end capacitance). But how to feed it? Since the ends of the 1/2 wave are high impedance, the bottom end needs to be fed with a high impedance source. A common and effective way to do this from a 50 ohm feed source is to use a 1/4 wave helical coil where the physical location of the end of said coil and the bottom end of the longer whip are physically located to give the proper capacitance between them so as to couple the two high impedance ends together efficiently. This is the classic pull up antenna on circa 1990 vintage cell phones. At GMRS FREQS, that coil would be maybe 1 inch long (it was 1/2 inch on the referenced 800MHz cell phones), and with nearly zero extra length for the coupling between the two sections. This would feed the 1/2 wave whip nicely. The result would be a whip about 13 inches, let's say maybe 14 inches with the SMA connector on the bottom. But to be clear: This will get you a 1/2 wave dipole radiation pattern and efficiency, nothing more, certainly not the 3dB gain advertised by some after market vendors. It's a method to recover from the lousy OEM dual band and/or short talkie body situation. But it does sacrifice VHF performance pretty severely. There is a subtle advanage where in this situation the ground currents on the body of the talkie are not as high (as the OEM case) and thus there is less loss from the hand absorption when holding the talkie. Hope this helps again