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Antenna Question

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mgolden2

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Good evening, everyone. I have a question regarding the antenna on an XG25 and XG75. My department recently purchased a large quantity of these radios (trunked, P25 800 MHz for use on the Kansas City MARRS radio system) and we were told by our radio shop person that the hi-gain antennas (the 6" antenna) must be used because "the radio is specifically tuned to that antenna". On our old P7100's, we had the short, stubby antenna and they worked just fine. A couple of users kept that antenna and put it on their new XG 75 and the radio shop guy is having a fit over this saying the stubby antenna will cause problems with the radio and cause the radio to lose functionality. Is this guy right? Is each XG 75 radio "tuned" to the high gain antenna attached to it?

Here are the antennas in question: https://www.harris.com/solution/xg-75pxg-75pe-two-way-portable-radio-antennas

Thank you in advance for any input and advice you may have.
 

snoopyII

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Yes, to a certain degree he's right, one's a 1/2 wave the other is a 1/4 wave. The radio was probably factory tuned using the 1/2 wave. Also, the old P7100 antenna is designed for 800 mhz only. The XG75 is a 7/800 mhz radio, and I see that there is a little bit of 700mhz on the system. It shouldn't affect the Rx side of things, you may just see some degradation on the Tx side. Sounds like your shop guy may be blowing it a bit out of proportion though, its not as if you're going to blow the radio up or anything.
 

Rred

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Changing from 1/4 wave to halfwave (or vice versa) is definitely going to create an impedance mismatch during TX and that's going to cause a radio to either power down or burn up, aside from not putting out full power. Yes, mismatched antennas can "work", many things can "work". I once found out that a friend's car had literally eaten the tips of two spark plugs, but the other four were letting that engine "work" just fine. Worked a whole lot *better* once all six were properly replaced.(G)
 

prcguy

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This is just my opinion since I don't know the innards of the radio in question, but I seriously doubt there is any custom tuning in the radio to match a certain antenna, that would be a production nightmare for the mfr. I would think the radio is designed for a 50 ohm antenna and its the antenna that should match 50 ohms whether its a 1/4 wave or 1/2.

All 1/2 wave rated antennas I've seen for the 700/800 range plus cell phones are a 1/2 wave coaxial dipole, which have a reasonable match to 50 ohms. I've never seen a 1/2 wave end fed which is very high impedance.
prcguy
 

Rred

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I'd suspect part of the confusion could come from the radio shop trying not to confuse the customers with technical terms. For instance "tuning" the radio might just refer to impedance matching, which no sparkie would call tuning, but that's still a good way to explain impedance matching to a non-technical customer.

then there's "half wave" which can also be geekspeak, since a conventional "half wave" dipole is actually two 1/4 wave pieces, which in an h/t would mean a 1/4 wave antenna plus the counterpoise. So, the antenna could be a 1/4 wave helical...and still be half of what is really a half-wave antenna. Which would have something like a 70 ohm native impedance, not 50 ohm. Not a big impedance mismatch, not a critical one, but if the factory built the radio for a 50 ohm antenna impedance and matched that, then using a 70 ohm or vice versa antenna would push the SWR from 1:1 to 1.4. Certainly tolerable, but arguably "wrong".

Just saying, there are some fairly obvious places where "the guy said" stuff that got lost in translations. But still, the basic technical point (that there's a right and a wrong antenna) remains.

Hey, Sir Isaac Newton said my toast falls butter side down because of gravity. My dog says no, it falls down because he can't reach it on the table. Doesn't really matter which one was right.(G)
 

nd5y

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I've never seen a 1/2 wave end fed which is very high impedance.
I have. I worked for a cellular handset manufacturer about 20 years ago. This was back when everything was 869-894/824-849 MHz.
One product line had the antenna connector on top. They shipped with a 1/4 wave helical and had an optional "high gain" antenna which was a 1/2 wave that had a small (about 1/4 x 3/4 in.) PCB in the base with some coils and caps on it. Another line had a built in flip up helical antenna and a connector for the optional 1/2 wave antenna.
 
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prcguy

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What I meant to say was I've never seen a 1/2 wavelength long wire antenna used with handhelds, etc, with no matching at the base. Some of the previous conversation hinted at a 1/2 wave antenna with a different enough match to consider special tuning inside the radio to compensate. To me that would point to a half wave end fed with no compensation at the base of the antenna, which would be a disaster on a typical hand held transceiver.
prcguy

I have. I worked for a cellular handset manufacturer about 20 years ago. This was back when everything was 869-894/824-849 MHz.
One product line had the antenna connector on top. They shipped with a 1/4 wave helical and had an optional "high gain" antenna which was a 1/2 wave that had a small (about 1/4 x 3/4 in.) PCB in the base with some coils and caps on it. Another line had a built in flip up helical antenna and a connector for the optional 1/2 wave antenna.
 

DisasterGuy

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So much bad info here. All Harris radios factory ship with a 1/4 wave antenna unless specified otherwise. Harris designs their systems for portavle coverage on a 1/4 wave.

The system in question is a Motorola system. Motorola delivers their portable radios with 1/2 wave antennas unless specified otherwise. Likewise, Motorola designs system coverage for a 1/2 wave antenna.

What the shop is telling you is that the system is designed for portable coverage assuming a 1/2 wave antenna. If you use a 1/4 wave (Harris standard) antenna you will not have coverage per system specification. Several systems struggle with this including Delaware where a fire chief must sign a waiver indicating that the decision to use an antenna with performance under system design is at their sole liability and could result in failure to communicate leading to responder fatality.

Sent from my SM-G900V using Tapatalk
 

RFI-EMI-GUY

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The Motorola 1/2 wave antennas I am familiar with are coaxial design, center fed 1/4 over 1/4 wave. They are a superior antenna due to elevated feed and the tuned counterpoise. Impedance is not a problem with this design. It is internally tuned to match the 50 ohm radio impedence.

Users complained they were too long, but in fact using a short 1/4 wave end fed is a terrible compromise. If this is for public safety, follow the recommendations of the system owner.

The short ones might "work fine", but doing an AB comparison in a fringe reception area will quickly show that size matters.

Sent from my SM-T350 using Tapatalk
 
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jim202

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Mar 7, 2002
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New Orleans region
I run into this long verses the short (stubby) antenna all the time. Yes there is problems with using the short antenna on your portable radios. The short antennas do not provide the needed coverage out on the fringes and inside buildings with poor inside radio coverage.

But the real issue here is the officer complaints about the long antenna poking them in the arm pit. Seems they don't care about the radio working well out on the fringes or in buildings with poor coverage. All they want is for the antenna not to poke them in the arm pit.

Not what I call a wise decision by those depending on their radio in an emergency.
 

kb4cvn

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Portable antenna 101:

Any portable radio antenna less than ¼-wavelength in length is nothing more than a radiating dummy load at best. 50 Ohms to keep the radio fat, dumb and happy.


FORMULA: 234/frequency in MHz = length in feet, or 2080/frequency in MHz = length in inches


=================

For 800 MHz,
2080 / 800 = 2.6 inches = a quarter wave whip.

A quarterwave whip over a proper groundplane exhibits a gain of Ø dB.
1 watt in, 1 watt radiated, life is good!

At 800 MHz, this is the little 3 inch whip with the YELLOW stripe cap you see most common.

At 800 MHz, you can also encounter two other models of longer antenna:
A coaxial ½-wavelength dipole. (this is the yellow cap whip, ~9 inches long, tapers halfway to skinny)
And an end-fed ½-wavelength whip. (this is the whip around 8 inches long, fat at the bottom for a matching coil and skinny at the top)

Both of these ½-wavelength whips exhibit a gain of 2 to 2.5 dB. Yes longer, but it beats not having coverage.

If you are using a helical whip on 800 MHz (1.5 inches or less), the LOSS FACTOR on these is -6 dB to -9 dB. For a shorter length, you are giving up 2/3’s of your transmit and receive range.

(Try explaining that to a Cop, and watch their heads explode !!!)

Broadband portable radio antennas for 700/800 MHz are similar in most cases, for this simple overview…

=================


Let’s jump down to VHF-Highband and UHF frequencies.

At VHF-HIGHBAND, a quarter wavelength whip is roughly 17 inches, depending on exact frequency, and offers a gain of Ø dB.
For most applications, the standard antenna is the helical whip (aka: rubber ducky) at a length of ~6 inches. Like at 800 MHz, the LOSS FACTOR on these shorter antennas is -6 dB to -9 dB.

Some people even opt for the “stubby” helical whip at VHF-HIGHBAND, and go with a 2 inch model. You can expect a -20 dB LOSS or worse with these.

=================

At UHF frequencies, a quarter wavelength whip is roughly 6-7 inches, depending on exact frequency and offers a gain of Ø dB

For some applications, the antenna of choice is the helical whip (aka: rubber ducky) at a length of ~3 inches. Like at VHF-HIGHBAND & 800 MHz, the LOSS FACTOR on these is -6 dB to -9 dB.

What is really bad with these antennas is that Public Safety users will be issued a Speaker-Microphone with remote antenna fed with a three-foot section of incredibly lossy RG-174 coax then attach a 3 inch helical whip.

The net result of this is a few watts, fed though seriously lossy coax, then connected to an inefficient helical antenna were only 1/3 of the effective radiated power is getting out.


=================

And we wonder why users complain about portable coverage?

My humble 2¢ worth....
 

KG7PBS

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Mar 19, 2015
Messages
401
Location
Sacramento CA
Reno and Washoe county LE uses Harris XG75 with the Stubby 700/800 MHZ antennas. And they are what is given to the officers from the Radio Shop. Its the same one i have on my Harris P7200 now its just fine. RX only no TX
 
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