Custom Modified 700 MHz Scanner Antenna

[Joseph11]

As a random project, I decided to modify a cheap/unused 144/430 MHz Nagoya NA-771 antenna I had lying around to work best on 700 MHz. I cut the element down to about 7.27" to perform ideally on 772 MHz (middle of the 700 MHz repeater output range) as per KD4SAI's Antenna Calculator. Question is, does this modification make sense technically? I'm not concerned about SWR since it'll be used to receive only. What's in the base of the antenna? Would it affect how I should have cut it? Was this the proper way to modify this antenna to work better on 700 MHz? Is it even logical to use this antenna for such a project? Thanks.

 

[br0adband]

If you already cut the antenna there's no turning back now - if you have not cut the antenna then STOP and don't bother, you're not going to make much if any difference whatsoever by damaging it in that manner. It seems based on reading your post twice that it's already too late so, personally if you really need to get better 770 MHz reception I would have simply said either purchase something like this:

700-800MHz Antenna For Motorola XTS1500 XTS2250 XTS2500 XTS3000 Portable Radios | eBay

and the necessary adapter for a few bucks more since it's designed to work in that range and covers 800 MHz as well, I think the center frequency it's cut for is about 820 MHz to be centered in the 768-870 MHz range. Or buy one of the awesome Radio Shack 800 MHz duckies, or the clone of it made by Remtronix these days, either of those should work fantastic for 768-870 MHz operation, I know mine does - I have a Diamond RH77CA like many many other people do and that receives the 770 MHz range fine in my experience and that's what that Nagoya NA-771 happens to be a clone of.

"Rubber ducky" antennas aren't specifically working the way they work because they're measured to a certain physical length meaning just because it's physically 14.7" long and you cut it in half to about 7.3" long it's not necessarily going to perform better on the frequency that 7.3" might be a 1/4 wavelength at - in fact, a 7.3" antenna would be basically a 1/2 wave element at 770 MHz almost precisely, go figure. Sometimes such antennas could have a loading coil assembly inside and just cutting it at some point may wreak havoc with the reception capability after the modification.

All I can say about it is this: if you already cut it, and you're getting better reception at the 770 MHz comms you were hoping to receive better then bonus, you are fortunate, but in the future if you're thinking about modifying a pre-built "rubber duckie" - and I use that term to mean any standalone antenna that's designed to be used with a handheld scanner or portable unit that doesn't involve additional hardware/cabling/etc and isn't a telescopic adjustable whip - just by grabbing some wire cutters and snipping it it as some point in it's length based on the numbers I'd recommend against it and instead purchase something that's pre-built for that length/frequency.

 

[Joseph11]

I appreciate the detailed reply, br0adband. It was just an experiment on a cheap antenna I never really used, so I'm not concerned about the loss of the antenna. It definitely receives better on 700 MHz than it did prior to being cut, but I see little difference between it and the RS 800 MHz antenna. I'm primarily looking for something centered in the 700 MHz range rather than the upper 800 MHz range for receiving. Does such a thing exist with a BNC connector?


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[br0adband]

I've seen some really cheap ones on eBay recently that claim to be 700 MHz tuned (so about 770 MHz give or take a few I suppose) but honestly I wouldn't even bother as the RS 800 works just fine for my purposes. That's not to say that an antenna designed specifically for 770 MHz center cut operation doesn't exist, but if one does I've never found it anywhere in my searches for one.

The reason that such an antenna doesn't exist is pretty straightforward: the whole idea behind the more recent move towards the 700 MHz spectrum is two-fold - the first aspect being that the frequency band happens to be available, and the second is more towards interoperability between agencies that might choose to use different frequency bands/hardware but still be able to communicate with each other.

That second aspect would more than likely be the main reason you won't find an antenna specifically designed/tuned for 770 MHz operation. Why? Because interoperability would require such hardware to be operable on both the 700 MHz band and also the 800 MHz band and because of that dual operations nature you'll only find antennas that are cut at about 820 MHz as I mentioned above. The 700 MHz band is roughly 769 to 775 MHz (and also 799 to 805 MHz) and since the well known and well established 800 MHz band (851-869 MHz originally but in years past because of rebanding that was reduced to 851-861 MHz) to have an antenna that works equally well or about as equal as it can be across those two ranges means it'll be cut for the frequency smack int he middle of the lowest and highest so 768 and 869. Manufacturers more than likely didn't alter the upper frequency limit in the math so the antennas are still looking at being cut to a center frequency of about 820 MHz or very close to it so they cover both bands very well.

The RS 800 is so old that it was cut for about 860 MHz - it was first produced way back when that 851-869 MHz spread was fully utilized in many public safety agencies but the rebanding changed that and again they wouldn't scrap the design of that antenna and recut/retool it just to accommodate a minor difference in the receive range - as you noted the SWR aspect isn't relevant due to it not being designed for transmitting but I'm sure some folks have tested that in the past.

If you need a really good 772 MHz centered antenna I'd say make one yourself: a simple 1/4 wave ground plane antenna like this would work wonders compared to most any rubber duckie you'll ever find:

Now, obviously that's not a BNC connector there but you could use a piece of coax to go between the scanner and the antenna obviously. I'm a big fan of making homebrew antennas like that and it's coat hanger sections (or #12-14-16 gauge electrical wire) cut to the 1/4 wavelength (the ground elements are slightly longer which is what I recommend) attached to an SO-239 chassis mount with spade lugs and screws and nuts - the ones I make aren't even soldered, there's just no reason to do that in my opinion as that would gain such a minuscule amount of signal it's not worth my time to do it and I like mine being "adjustable" in the sense that I can disassemble them quite easily if needed.

Anyway, I don't know if you have a handheld or a base/mobile model but I'm going to presume that it's a handheld and you'll want to stay portable for whatever reason so these ground planes are impractical for that purpose. If however you want something that you can use at home or wherever you might happen to be in a more static/stationary sense, building one of these cut to 772 MHz (so the main vertical element would be just shy of 3.65 inches tall and the ground elements make 'em 4 inches exactly) would absolutely provide a big difference in reception over any duckie you might happen to find.

If you absolutely have to stick with a duckie-style antenna and be mobile, the RS 800 or the Remtronix clone (BNC models) are still the recommendation if you wish to not use an adapter to get that Motorola antenna working. That Motorola would technically offer better reception for 700 MHz reception because of the lower center frequency it's cut for.

Hope this helps...

 

[Joseph11]

I would need an adapter like this for that Motorola antenna, correct? Would you recommend that specific one or a different brand? Would loss from the adapter outweigh the benefits of using that antenna over the RS 800 MHz antenna?

 

[br0adband]

That looks to be pretty much the exact connector, yep, it'll look a little weird I suppose given the Motorola antennas have "bell bottoms" but as long as it works that's what matters most. As far as possible insertion loss because of the adapter I can't say for sure - yes technically there will be a loss there but if I had to say I'd simply reference this info from Amphenol:

What is Insertion Loss and how is it specified? | Amphenol RF

The relevant info:

In general, the insertion loss of a connector is on the order of a few hundredths to a few tenths of a dB.

Basically I'd call the insertion loss irrelevant but some might argue otherwise and I'd answer that nobody is going to notice that level of loss without hooking up measuring equipment. That amount wouldn't even be equal to 1/10th of 1 bar of signal strength on a 5-bar scale so, having the antenna tuned to the lower center frequency would more than offset any potential insertion loss.

So yeah, that's the connector and that's the antenna I'd couple it with but as a last word I'll say this: I can't really imagine you're going to improve things all that much over using the RS 800 or what you created from that Nagoya antenna but it's not like a ton of cash out of pocket to get the Motorola antenna and the adapter.

 

[Joseph11]

Thank you for all the detailed information! I've ordered the antenna and adapter.

 

[Joseph11]

I ended up getting the antenna you linked, br0adband, and this adapter. Melted some heat shrink tubing over the adapter and ended up with the product shown in the photos below. I've noticed a bit of improvement with this over the RS 800 MHz antenna, especially with a local 700 MHz P25 simulcast system, but slightly lower signal strength on bands below 700 MHz (which is fine since I'm focusing mainly on 700 MHz, anyway). I'm quite happy with the result. Thank you again for your assistance with this!

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