Weird picket-fencing on UHF freq ONLY

[LIScanner101]

1 /4 wave whip, 5 / 8 wave whip, 5/8 wave over 5/8 wave, collinear etc…..for a "scanner only" buff this is confusing to me but the understanding of what's best in certain situations might help me understand this phenomenon. I have noticed when I listen to the typical scanner bands on long antennas such as the Antenna Specialists MON- Series (which seem to give me the BEST coverage OVERALL so far) I tend to experience the following:

VHF low (my range is 33MHz to 46MHz): Base stations usually very clear to excellent, mobiles choppy depending upon how close (either always clear or always scratchy).

VHF high (around 155MHz): ALMOST ALWAYS excellent. VERY little choppiness or picket fencing. Only noise depends upon signal strength (either always clear or always scratchy).

UHF (450-480MHz typical). Strange! I hear a LOT of picket fencing on this band and it doesn’t seem to follow “the closer the better rule”. It seems like there is no pattern as to whether or not I will pick this band up well or not. NOTE: I am NOT talking about “scratchy” signals that seem distant. What I am referring to is, if I am stopped in traffic, I can pick up Anytown FD clear as a bell on UHF, but as soon as I start driving, it gets choppy. This RARELY happens on low band, high band or 800MHz.

800MHz: Almost always clear, very little choppiness. This surprises me.

I have tried this with all my MONs. With the DPD 5/8 over 5/8 800MHz antenna I 800MHz is of course EXCELLENT, VHF high is very good, low band suffers a little but UHF STILL picket-fences. Even with the “venerable” Laird tri-band that many people love, UHF still suffers:

My conclusion: So far, my best antenna overall seems to be my MON-Series, but even with those, UHF seems elusive.

Is there something about UHF that makes it prone to picket-fencing?

Would going to a simple ¼ wave or a 5/8 over 5/8 dedicated UHF help?

 

[K5MPH]

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Let us know where your antenna is located.....

 

[prcguy]

Picket fencing is generally not caused by weak signal but rather plenty of signal that has bounced and arrived at your receiving antenna out of phase causing a cancellation. You can even be close to the transmitter site with the transmit antenna visible but with something like a large metal water tank in between but off to the side that is providing a strong reflected signal that is delayed in time, although it may only be measured in nano seconds.

In these cases changing from a low gain to high gain receive antenna may not make any difference in picket fencing. Some areas just have too many reflective objects to overcome and the transmit is probably not terribly high, which may add to the problem.
prcguy

1 /4 wave whip, 5 / 8 wave whip, 5/8 wave over 5/8 wave, collinear etc…..for a "scanner only" buff this is confusing to me but the understanding of what's best in certain situations might help me understand this phenomenon. I have noticed when I listen to the typical scanner bands on long antennas such as the Antenna Specialists MON- Series (which seem to give me the BEST coverage OVERALL so far) I tend to experience the following:

VHF low (my range is 33MHz to 46MHz): Base stations usually very clear to excellent, mobiles choppy depending upon how close (either always clear or always scratchy).

VHF high (around 155MHz): ALMOST ALWAYS excellent. VERY little choppiness or picket fencing. Only noise depends upon signal strength (either always clear or always scratchy).

UHF (450-480MHz typical). Strange! I hear a LOT of picket fencing on this band and it doesn’t seem to follow “the closer the better rule”. It seems like there is no pattern as to whether or not I will pick this band up well or not. NOTE: I am NOT talking about “scratchy” signals that seem distant. What I am referring to is, if I am stopped in traffic, I can pick up Anytown FD clear as a bell on UHF, but as soon as I start driving, it gets choppy. This RARELY happens on low band, high band or 800MHz.

800MHz: Almost always clear, very little choppiness. This surprises me.

I have tried this with all my MONs. With the DPD 5/8 over 5/8 800MHz antenna I 800MHz is of course EXCELLENT, VHF high is very good, low band suffers a little but UHF STILL picket-fences. Even with the “venerable” Laird tri-band that many people love, UHF still suffers:

My conclusion: So far, my best antenna overall seems to be my MON-Series, but even with those, UHF seems elusive.

Is there something about UHF that makes it prone to picket-fencing?

Would going to a simple ¼ wave or a 5/8 over 5/8 dedicated UHF help?

 

[wtp]

imo

fence picket is normal if you are moving, so i guess you are not
it could be that the transmitter is moving, also i guess not
it could be that cars or trucks are in between your antenna and the transmitter or
a loose squelch on your radio. my old pro2004 would do what you are saying and there was a resistor that was changed and all was well
so another question is what radio does this happen to?

 

[LIScanner101]

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Let us know where your antenna is located.....

When I travel its always on the trunk lid with an NMO mount. However, I have a Ford F250HD that has an NMO right in the middle of the roof and a run of LMR-300 coax and I've heard it even on that setup.

 

[LIScanner101]

fence picket is normal if you are moving, so i guess you are not

Please see my original post, I mention that this only happens when I'm moving and only on UHF.

it could be that the transmitter is moving, also i guess not
it could be that cars or trucks are in between your antenna and the transmitter or a loose squelch on your radio. my old pro2004 would do what you are saying and there was a resistor that was changed and all was well so another question is what radio does this happen to?

This has happened on a PRO-163 as well as what I use now, which is the Uniden BCT15X, but again, ONLY on UHF. Did you experience this only on UHF as well or on all other bands?

 

[LIScanner101]

Hmmm, that kinda sucks ....

So this may be something I have to live with on UHF? Maybe I'm getting TOO much signal and should try a 1/4 wave?

Picket fencing is generally not caused by weak signal but rather plenty of signal that has bounced and arrived at your receiving antenna out of phase causing a cancellation. You can even be close to the transmitter site with the transmit antenna visible but with something like a large metal water tank in between but off to the side that is providing a strong reflected signal that is delayed in time, although it may only be measured in nano seconds.

In these cases changing from a low gain to high gain receive antenna may not make any difference in picket fencing. Some areas just have too many reflective objects to overcome and the transmit is probably not terribly high, which may add to the problem.
prcguy

 

[wtp]

oops

sorry i did not read the mobile part
as for my radio, it was all signals
you could break squelch and then turn it past that point to get quiet again
loose squelch or hyper something, too long ago and was covered in bill cheeks books
but not the same problem

 

[LIScanner101]

Thanks wtp.

Well, the only thing I can imagine being my problem is this:



-VHF low and high come in with almost no picket-fencing because their wavelengths are longer and not as affected by multipath

-800MHz, while intuitively a “worse” frequency than UHF, comes in clear because all of the 800MHz stuff I listen to is trunked and the antennas being used are all typically located in very tall towers (much higher than the typical PD/FD tower)

-UHF “picket fences” because, being shorter wavelengths than VHF, it is much more prone to multipath AND the UHF antennas on most PD/FD’s I listen to are located on top of a PD or FD station, and are not as high above the ground as a typical 800MHz TRS antenna.

-OR-

I could be just rationalizing the whole thing and I have no idea what I am talking about :roll::lol:

 

[wtp]

sounds right

some folks come here and pose a question and
they themselves find the answer after putting it in "writing"
you are the best at find things out because you are there!
what you said does sound logical, i agree with it for what it is worth
enjoy the hobby.

 

[prcguy]

Its not from too much signal. When you have reflections and multipath it can create fades of 20 or 30dB when the reflected signal arrives 180 deg out of phase or in other words its delayed by exactly one half or multiples of one half wavelength.

One characteristic of multipath is the fades are spaced one half wavelength apart and at UHF you would experience a fade every 1ft your that car moves when in an area where reflections are strong enough to interact with the main signal.
prcguy

Hmmm, that kinda sucks ....

So this may be something I have to live with on UHF? Maybe I'm getting TOO much signal and should try a 1/4 wave?

 

[rfradioconsult]

The correct term is Rayleigh fading, as others have posted changing antennas would have minimal impact or improvement. Both 450 and 800 MHz can be affected more dependent on factors such as the type of area (buildings, terrain, etc.). Commercial radios have better robust receiver designs and are able to handle the rapid change in RX signal level better. Back in the mid 80's when cellular was starting to be rolled out the mobile units were designed to use two antennas to provide diversity receive to improve receiver performance but users didn't like having two antennas on their vehicles so the idea of two antennas never became a standard.

 

[902]

Back in the mid 80's when cellular was starting to be rolled out the mobile units were designed to use two antennas to provide diversity receive to improve receiver performance but users didn't like having two antennas on their vehicles so the idea of two antennas never became a standard.

And that's unfortunate, because those were the more reliable transceivers, at least before AMPS and subsequent digital sites became over-built and heavily compressed to the point of diminished audio quality (in my view, anyway).

You might want to bring up that this isn't all "bad." Modern technology that pushes heavy loads of data is actually enhanced by multipath and uses multiple antennas (MIMO).

Here's a very techy presentation from Lon-giland with neat pictures of the concept (and math!).
http://www.ieee.li/pdf/viewgraphs/wireless_mimo.pdf

 

[LIScanner101]

The correct term is Rayleigh fading, as others have posted changing antennas would have minimal impact or improvement. Both 450 and 800 MHz can be affected more dependent on factors such as the type of area (buildings, terrain, etc.). Commercial radios have better robust receiver designs and are able to handle the rapid change in RX signal level better. Back in the mid 80's when cellular was starting to be rolled out the mobile units were designed to use two antennas to provide diversity receive to improve receiver performance but users didn't like having two antennas on their vehicles so the idea of two antennas never became a standard.

Thank you for the additional details. Oddly enough, as I mentioned above, I don't have nearly the same magnitude of problems on any 800MHz frequencies as I do on UHF. I would have thought that "the higher the worse" but that hasn't proven out to me in the real world. Does this mean my scanner has a better RF section on 800MHz than it does on UHF?????

To your other comment about using multiple antennas: I assume it's not going to be as simple as merely slapping on an identical antenna to my vehicle and connecting them together with a combiner/diplexer, right? That almost sounds too easy and good to be true...

 

[LIScanner101]

And that's unfortunate, because those were the more reliable transceivers, at least before AMPS and subsequent digital sites became over-built and heavily compressed to the point of diminished audio quality (in my view, anyway).

You might want to bring up that this isn't all "bad." Modern technology that pushes heavy loads of data is actually enhanced by multipath and uses multiple antennas (MIMO).

Here's a very techy presentation from Lon-giland with neat pictures of the concept (and math!).
http://www.ieee.li/pdf/viewgraphs/wireless_mimo.pdf

Lon-giland...LOL!!! Us "islanders" prefer "Lawn Guyland" (at least that's how it sounds when I say it :lol: )

Thanks for the additional data, this is all good stuff and helpful.

 

[902]

Lon-giland...LOL!!! Us "islanders" prefer "Lawn Guyland" (at least that's how it sounds when I say it :lol: )

Thanks for the additional data, this is all good stuff and helpful.

I've been trying to get it right :wink: After I moved "down south," I'm in Little Massapequa.
Most of my neighbors are from Long Island. We got that Devils - Islanders thing going on. LOL!