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Thread: Motorola Portable Antennas

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    Default Motorola Portable Antennas

    Hey all, I was looking at antennas a while back and saw the list of antennas and bands for the XTS5000. I currently have the nad6566 VHF (136-150.8Mhz). MY question is has anyone ever been able to run one on a analyzer to see how far the band goes? I run a mix between 145Mhz and 155Mhz. So how bad does the antenna get after the 150.8 mark? Thanks!


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    Usually, the "advertised" bandwidth of an antenna is the width between the 3dB points. Understand that as plotted on something like a Spectrum Analyzer, the antenna doesn't have a sharp operating region like a notch. It's a smooth curve going from some optimal center frequency point where it's at its maximum gain, then as you go away from that point in either direction the gain will drop. The point where the signal drops by 3 dB (1/2) is one edge (high or low frequency) of the range. Repeat on the other side to find the other 3dB point.

    Having said all that, the antenna's smooth gain curve continues beyond the 3dB points and continues to lose gain, but still exhibits SOME. Hence a low cut 138-151 MHz antenna will still kind of work at 162 MHz. It won't be deaf, but it won't be great and the gain will be less than the published gain at that frequency. The only way to tell for sure is to plot it out with a VNA or an antenna analyzer, and even that doesn't give the full picture, because the radio frame and your hand and arm on it form a counterpoise or ground plane for the antenna to work against which completes the image, but won't be present if you try to connect a portable antenna to an analyzer. It would be like testing a ground plane 1/4 wave antenna with just the vertical and no ground plane. Instead of 50 ohms that antenna will probably look like an end-fed vertical and will be high impedance, like 1200 ohms or so, and will be correspondingly crappy in operation.

    To accurately map and measure the gain of an antenna, you really need to set up a test range with a test jig that rotates or you have to go in circles taking level measurements at different frequencies to find out what the reality is for gain and pattern measurements.

    Bottom line is you can get an approximate gain trace with a microVNA or better analyzer, but it won't be 100% accurate no matter what. This is one of the reasons why antenna design, especially for portables is PFM (Pure F'n Magic) and why so many designs are just approximating garbage that "kind of works". But the microVNA is good enough to give you an idea of where the rough 3dB points are, plus they only cost about $50 bucks. I recently mapped out the curve of an APX8000 5-band antenna and you could see all the dips in the curve at the VHF, UHF, 800 MHz frequencies. It wasn't very stable and getting your hand near it disturbs the field and the readings change. As I said, not optimal, but it works.

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    About a year ago, I did a test on my XTS2500 comparing the "wide band" Motorola VHF antenna against the low split one you describe. Had myself at a very fixed spot, and talked thru a QUANTAR on 147 mhz. Repeater is about 12 miles TRUE LINE OF SIGHT. The low split Motorola antenna was 3 dB better than the Motorola "wide band" per the Astro-TAC. Did a similar test at 155 freqs on a similar system. And again the low split antenna performed better to the input of the repeater system. So I quit using the "wide band". Now, in the 160s I have no idea. Maybe the wide band is better. I kinda doubt it tho. Who knows if the transmitter "likes" the RL of the wide band in 155. I just found its performance at the freqs I use most to be disappointing. Even the correct 155 split antenna did not work as well as the low split one on 155. Not my first rodeo, so did my best to make sure nothing in the near field changed when swapping antennas. YMMV.

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    I will add that a friend and I have been doing a LOT of measuring of base(repeater), mobile and handheld measurements utilizing drones and "fixed" measurement schemes for a few years. So, not making a lot of the usual misteakes. Still does not mean that your results and ours would be the same. There are way more variables than people (even experienced people) might think. At some point I think we all just choose the compromise that works best for our application.

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    Exactly. Nearly everything in life is a compromise. Most crappy dual-band portable antennas are just single band cut for 350 MHz, a compromise of working equally crappily on both bands.

    Gain versus Bandwidth is a compromise trade-off. As you noticed the narrower band antenna has higher gain. You have to trade one for the other, it's similar to the effect of a directional antenna in that you are getting higher gain in one direction, but at the expense of less gain in the other directions.

    If you don't need expanded coverage up above 155-160 MHz (like Marine or WX) then the narrower band antenna is the best solution. The improved gain is why they offered them for sale even after the wideband armpit-poker was invented. That and the fact that there were like an inch shorter than the Old Red Fishing Pole.

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    I did a comparison between this and a couple of other antennas a few years ago, that might be helpful here.

    https://communications.support/threa...nna-comparison

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    I'll have to post pictures later but I threw some of the more popular antennas on a VNA to see what the story is.

    Generally, the helical antennas perform fairly well in their prescribed band-split. More or less, you can expect 1.5:1 or better in the published specs, and rising very quickly outside them. For example, IIRC the 136-151 helical is something gross like 6:1 at 165MHz.
    The wide band antenna isn't as good as a split-specific, but is "overall good", varying from 3:1 at the bottom and 1.75:1 at the top, with the sweet spot being around 168.500MHz.

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    Here's a comparison photo of the "136-178 MHz wide band 203 mm red (NAD6563)" and the "151-162 MHz heliflex 183 mm black".
    You can see, the wide-band isn't particularly good anywhere but not particularly bad overall, whereas the band specific one is really bad out of the published spec, but quite good inside it.

    Keep in mind of course, this is on a "hobby grade" VNA, without a proper ground. Of course, on VHF you're not going to get much of one on the radio either.

    2019-11-27 23.47.18.jpg
    2019-11-27 23.57.04.jpg

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    One thing to take into account when sweeping portable antennas is the ground plane. or lack of one. I've tried in the past sweeping portable antennas both on and off the radio. I used an old radio and wired a piece of RG174 into the radio housing connecting it the the antenna base (after isolating it from the radio) with the other end to a Sitemaster.

    If found several megs of frequency shift depending if the sweep was done on the radio or off on some type of ground plane that I thought approximated the radio ground plane.

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    Quote Originally Posted by RFguy View Post
    One thing to take into account when sweeping portable antennas is the ground plane. or lack of one. I've tried in the past sweeping portable antennas both on and off the radio. I used an old radio and wired a piece of RG174 into the radio housing connecting it the the antenna base (after isolating it from the radio) with the other end to a Sitemaster.

    If found several megs of frequency shift depending if the sweep was done on the radio or off on some type of ground plane that I thought approximated the radio ground plane.
    Yes, I mentioned exactly that in post #2 above. Your putting it in the frame w/coax is a better solution than no ground plane at all for sure, but it's even more complex than that and includes your hand and arm as part of the counterpoise, too.

    Also as you said the field shifts on/off radio; it will also change if you get near it with your body or hand and it will not only mess with the peak resonance point, but it will change the gain at many frequencies, affecting them differently all up and down the band. This is clearly visible when using a true VNA to sweep the antenna.