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Antenna frequency range tolerance

sjmark24

Member
Joined
Jul 22, 2014
Messages
21
Hello,

I was wondering how tolerant frequency ranges are on pre tuned antennas.

If I have a range from 150.5 - 158.5 would receiving on a frequency 0.7mhz over the top tuned range have that major of a decline?

At what point will the signal degrade so much that its time for a different tuning range? ie 2.0mhz 4.0mhz?

Thank you
 

nd5y

Member
Joined
Dec 19, 2002
Messages
8,040
Location
Wichita Falls, TX
Almost any VHF high band antenna (150 MHz) will have a bandwidth of several MHz.
It depends on the type of antenna, the frequency range it is designed and tuned for, if it is used for transmitting or just receiving and to some extent the type of radio used.
Usually the higher the gain the narrower the bandwidth.

Professional grade antennas made by reputable manufacturers have bandwidth specifications published. It is usually called the 2:1 SWR bandwidth.
 

sjmark24

Member
Joined
Jul 22, 2014
Messages
21
Almost any VHF high band antenna (150 MHz) will have a bandwidth of several MHz.
It depends on the type of antenna, the frequency range it is designed and tuned for, if it is used for transmitting or just receiving and to some extent the type of radio used.
Usually the higher the gain the narrower the bandwidth.

Professional grade antennas made by reputable manufacturers have bandwidth specifications published. It is usually called the 2:1 SWR bandwidth.
Thank you for the response, I have a specific antenna with the specs at this link 220-3an

It has a 1.6:1 SWR. I've noticed issues when this is used for a repeater, the rx is slightly outside of the rated range, the tx is perfectly within. The issue I noticed is that portables can be heard within 5 miles and mobiles can be heard within 10 miles, but another base station at 15 miles will occasionally come in with rolling desense, at least thats what it sounds like. If the weather gets cloudy or humidity goes up then the station at 15 miles away will almost completely cutout on the rx side. Even if the tx is turned off.
 

prcguy

Member
Joined
Jun 30, 2006
Messages
8,006
Location
So Cal - Richardson, TX - Tewksbury, MA
That's a fairly high end antenna you have there. In my experience, using that antenna 700KHz outside its specified range, you will probably not be able to tell anything is different compared to using the antenna in the middle if its specified range. You might be able to tell with test equipment but not with radios in the field.

Several things will happen when you get too far outside its specified range, the match will creep up and eventually get to an unusable level. The performance will degrade, some due to loss in mismatch and some due to the elements being too far away from their resonant point. The antenna will also exhibit uptilt or downtilt of the radiation pattern when you get too far from its design point. If I remember correctly if you go too low in frequency the pattern will start to downtilt, and if you go too high in frequency the pattern will tilt above the horizon. Or is it the other way around....

Anyway, its very common for amateurs to use a commercial frequency range antenna like yours slightly outside its specified range with totally satisfactory performance in the amateur bands. The only way you might tell if its a problem is to do a side by side A/B test with the exact same antenna but in the proper frequency range and see if they both work the same or if one outperforms the other.

Thank you for the response, I have a specific antenna with the specs at this link 220-3an

It has a 1.6:1 SWR. I've noticed issues when this is used for a repeater, the rx is slightly outside of the rated range, the tx is perfectly within. The issue I noticed is that portables can be heard within 5 miles and mobiles can be heard within 10 miles, but another base station at 15 miles will occasionally come in with rolling desense, at least thats what it sounds like. If the weather gets cloudy or humidity goes up then the station at 15 miles away will almost completely cutout on the rx side. Even if the tx is turned off.
 

sjmark24

Member
Joined
Jul 22, 2014
Messages
21
That's a fairly high end antenna you have there. In my experience, using that antenna 700KHz outside its specified range, you will probably not be able to tell anything is different compared to using the antenna in the middle if its specified range. You might be able to tell with test equipment but not with radios in the field.

Several things will happen when you get too far outside its specified range, the match will creep up and eventually get to an unusable level. The performance will degrade, some due to loss in mismatch and some due to the elements being too far away from their resonant point. The antenna will also exhibit uptilt or downtilt of the radiation pattern when you get too far from its design point. If I remember correctly if you go too low in frequency the pattern will start to downtilt, and if you go too high in frequency the pattern will tilt above the horizon. Or is it the other way around....

Anyway, its very common for amateurs to use a commercial frequency range antenna like yours slightly outside its specified range with totally satisfactory performance in the amateur bands. The only way you might tell if its a problem is to do a side by side A/B test with the exact same antenna but in the proper frequency range and see if they both work the same or if one outperforms the other.
I splurged on this a couple years ago, just started using it for a different use. But I cant tell if the issues I'm having has to do with the rx being out of the specified range or if there is something else going on. It doesnt add up to me that someone using a mobile is able to hit the antenna clearly and easily from the same distance as the other base station but if that base station keys up the antenna it is good and then bad depending on the mood.

I hooked up a laird antenna that isnt tuned and is just a standard 136-174 antenna and that receives the base station just fine but cant pickup a mobile more than 5 miles away so I know that it definitely isnt receiving any better, just for some reason it can receive that base station better. I'm just stumped, unless its due to other antennas in the area causing inteference but only when I'm using the larger Commander antenna.
 

W5lz

Member
Joined
Feb 28, 2019
Messages
332
In general, the transmit frequency is the important one, receivers just aren't that 'picky' about impedance. If the transmit frequency falls within the usable range of that antenna, it -should- work just dandy. Most transmitters used for repeaters can be "scootched" (technical term) a little, usually not much though (adjusted). And there's always the feed line lengths you can play with. And then there's the duplexers you will have to deal with. Those duplexers always have some adjustment, but not usually for wide ranges. If you don't understand what you are attempting to do/use, I'd find someone who does. Repeaters are not all that 'simple' and you should be careful of the advice you get, including mine.
I posted this before your last post, so, will add a bit more. It sounds like there's a difference in antenna height, just for a quick guess.
 

mmckenna

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Jul 27, 2005
Messages
10,789
Location
SNCZCA01DS0
I splurged on this a couple years ago, just started using it for a different use. But I cant tell if the issues I'm having has to do with the rx being out of the specified range or if there is something else going on. It doesnt add up to me that someone using a mobile is able to hit the antenna clearly and easily from the same distance as the other base station but if that base station keys up the antenna it is good and then bad depending on the mood.

I hooked up a laird antenna that isnt tuned and is just a standard 136-174 antenna and that receives the base station just fine but cant pickup a mobile more than 5 miles away so I know that it definitely isnt receiving any better, just for some reason it can receive that base station better. I'm just stumped, unless its due to other antennas in the area causing inteference but only when I'm using the larger Commander antenna.

Could be your duplexer is out of tune.
Could be your coaxial cable is shot/corroded.
Could be your repeater is out of tune.

Poor receive could be caused by any of these.
 

Ubbe

Member
Joined
Sep 8, 2006
Messages
3,266
Location
Stockholm, Sweden
If I remember correctly if you go too low in frequency the pattern will start to downtilt, and if you go too high in frequency the pattern will tilt above the horizon.
That's correct. Usually also the SWR are better at lower frequencies than higher from the antennas tuned frequency. For receive of multiple frequencies I try to match an antenna to the higher frequencies, as lower frequencies will probably have better coverage than the higher ones. For transmit you are locked to the antennas best SWR but try and go low in frequency in the acceptable SWR range if the antenna can be tuned to a higher frequency. Any SWR could create problems in a duplex system.

You could try and extend the lenght of the ground radials as a test, if it can be done easily, to try to downtilt more as it could improve your coverage and that basestations signal should then also improve. But your findings seem to match how an antenna works at different frequencies.

/Ubbe
 

freddaniel

Member
Joined
May 6, 2011
Messages
87
Location
Newport Beach, CA
If you are not getting the range you expect from your handhelds, or your coverage varies with weather you might be suffering from moisture in your coax connector at the base of the antenna. I has been my experience one wrap of plastic tape is not enough to prevent moisture from entering the connector. Your antenna has an N-type connector and one drop of water will cause reduced performance. I suggest one wrap of quality plastic tape such as Scotch 33, then two wraps of 30-mil rubber tape usually used for wrapping high-voltage connections, like Scotch 130C or 2242.
Regarding your transmission line, for any full-duplex or repeater use, avoid coax like LMR400 as it will actually create receiver noise due to the shield design. Professional installations only use Heliax cable that has a solid copper jacket with plastic cover. Usually, 1/2 inch cable is acceptable under 120 feet length at VHF.
Also, when using duplexers it is strongly suggested only using jumpers made with double-shielded silver-plated cables such as RG-214 for large cable or RG-142 for small cable. AND it is essential you also use quality silver-plated connectors on all jumpers between the duplexer and the radio. "Leaky connectors never look bad"
Last, do not attempt to "tune" your duplexer without the proper test equipment.
To test your system for "desense," communicate from the repeater with a local microphone to a handheld at the edge of coverage, and listen to the handheld count to 20 on the local repeater speaker, while switching the "repeate disable" switch on and off [which simply turns the transmitter on and off]. If all is well, there should be not noticeable difference receiving a distant signal, regardless of the transmitter being on or off. If there is a difference, then some of your repeater's transmitter noise is finding it's way into the receiver and causing "desense," which reduces your coverage. Professionals usually use a signal generator with a variable output connected to another outdoor antenna at the repeater site to make this test. Modulating it with a tone makes it easier for quick tests.
 
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