Antenna length (gain) picket fencing.

[kidphc]

I read something that said that 6db antenna for 450-470 at about 31" picket fences more then a ¹/4 wave which is shorter and flops around less. Therefore exhibits less picket fencing.

 

Anybody have input or real life experiences?

 

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[axorlov]

7 minutes ago, kidphc said:

6db antenna for 450-470 at about 31" picket fences more then a ¹/4 wave

Not exactly the answer to the question you asked, however:

In my experience 11" BR-178 (which is 5/8) picket fences about the same as 31" BR-170 (which is 5/8 over 1/2). I do not see a difference in picket fencing, I do see a difference when stationary. 31" is tiny bit better.

[kidphc]

Not exactly the answer to the question you asked, however:
In my experience 11" BR-178 (which is 5/8) picket fences about the same as 31" BR-170 (which is 5/8 over 1/2). I do not see a difference in picket fencing, I do see a difference when stationary. 31" is tiny bit better.

Any answer is more insight then no answer.

Started wondering if picket fencing wouls be higher on the longer antenna due to it whipping around more and adding to the mulipath issue. Or even if the higher gain can cause issues with the ability to capture more signal, due to the surface area.

But your response would say that neither really would be a differential that contributes.

Thank you for your response. Curios what other operators have experienced.

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[marcspaz]

Picket fencing is not caused by the antenna movement. It's caused by characteristics of the terrain and the speed of the vehicle, compared to where the receiver/repeater is.  Weaker signals are more susceptible to it because there is not enough energy to scatter enough signal to overcome shadowing from trees and buildings. It's a power/line of sight issue.

[kidphc]

Picket fencing is not caused by the antenna movement. It's caused by characteristics of the terrain and the speed of the vehicle, compared to where the receiver/repeater is.  Weaker signals are more susceptible to it because there is not enough energy to scatter enough signal to overcome shadowing from trees and buildings. It's a power/line of sight issue.

So a longer antenna should experience less.

Trying to figure out if I want to use a 1/4 wave or a 5/8 over 5/8 on the roof. Put side of the 9" vs 31" lengths.

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[Lscott]

The problem with high gain antennas is the height. You have to constantly remember it’s up there. No garage parking, fast food and bank drive throughs and low hanging tree branches to name a few of the hazards. The first time you forget is when it gets ripped off the roof. If you’re lucky it’s just the antenna/mount that gets wrecked.

[tweiss3]

Yea, I run the 2/70B and it is very very tall, but at this point I'm not switching it back out for the 2/70SH. I will go back to the 2/70SH when I swap trucks and go permanent mount. I don't notice a ton of picket fencing on my side though, some say it happens when I'm in the valley, which makes sense, but its still fully copyable.

[KAF6045]

1 hour ago, kidphc said:

So a longer antenna should experience less.

Trying to figure out if I want to use a 1/4 wave or a 5/8 over 5/8 on the roof. Put side of the 9" vs 31" lengths.

Use of a GAIN antenna means your transmission (and receive) pattern is narrower in the horizontal plane. A 1/4 Wave over a ground plane (or 1/2 wave dipole mounted high enough to avoid ground interference) is essentially a balloon pattern -- less strength directly to the horizon, but more signal at higher angles. It may receive signals leaking /over/ obstacles when the gain antenna is blocked by the obstacle.

 

[marcspaz]

9 hours ago, kidphc said:

So a longer antenna should experience less.

Trying to figure out if I want to use a 1/4 wave or a 5/8 over 5/8 on the roof. Put side of the 9" vs 31" lengths.

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Not necessarily a taller antenna, but rather an antenna with a decent amount of gain (which are typically taller than a 1/4 wave).  The terrain, elevation changes and who you are talking to in relation to those conditions will determine what the best antenna for you would be.  Gain antennas are not the best choice in very hilly or mountainous areas, but are a great choice for relatively flat areas. 

[kidphc]

I have always wondered if the swr on the Comet 2×24sr was better then the 2/70b because of the fact the engineers took the antenna bending at speed into account. So a static swr test would show a better swr on the 2x24 then that where the 2/70 would perform better (compared to itself) with a dynamic load.

I have seen people claim that with a flexible antenna you can experience doppler effect from the antenna wagging in the wind. Personally, never experienced and fading in and out not even on a 7ft long mobile antenna. But who knows.

2/70sh is not cutting it in certain areas. I think it may be a signal shadowing more the multipathing issue. New properly pre-tuned antennas should be here today.

The repeater in question is in Wheaton, Md over 100' off the ground, is on a water tower. It has performance the rivals or exceeds several 2m ham repeaters in the area and is down on power even comparatively, due to power caps in their respective services. Not the repeater or ita gear (old commercial repeater hardware, it was built the way it should have been.)

I know the Mormon Temple, corresponding dip by it on the highway poses an intresting rf issue. Since point a (repeater) to point b the Temple sits between the two in almost a straight line, closer to point b (the highway). Recieve can be spotty but obviously transmit goes from scratchy to garbage to nada, as expected.


So the question was posted for more of a study of reality vs theory, in this case internet hearsay vs reality


When I get the proper 5/8th and 1/4 wave gmrs antenna I will report back.

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[KAF6045]

5 hours ago, kidphc said:

I have seen people claim that with a flexible antenna you can experience doppler effect from the antenna wagging in the wind. Personally, never experienced and fading in and out not even on a 7ft long mobile antenna. But who knows.

Might be difficult to detect, given that FM itself is sort of "doppler" -- the frequency is deviating around the center (tuned) frequency to carry the voice signal.

Now... AM (old CB with a 10ft 1/4wave whip bumper mounted) I could see doppler, as the carrier frequency is moving around (along with the side bands) it would show as a tuning error. SSB it would show as a change in voice pitch (as the offset from tuned frequency determines pitch, and if the sideband is moving en-bloc, the pitch will move with it)

[kidphc]

3 minutes ago, KAF6045 said:

Might be difficult to detect, given that FM itself is sort of "doppler" -- the frequency is deviating around the center (tuned) frequency to carry the voice signal.

Now... AM (old CB with a 10ft 1/4wave whip bumper mounted) I could see doppler, as the carrier frequency is moving around (along with the side bands) it would show as a tuning error. SSB it would show as a change in voice pitch (as the offset from tuned frequency determines pitch, and if the sideband is moving en-bloc, the pitch will move with it)

Ok. Don't think I have heard that. Or did I on 10m/11m i am messing with the clarifier a lot so could be the sideband shifting.

[KAF6045]

2 hours ago, kidphc said:

Ok. Don't think I have heard that. Or did I on 10m/11m i am messing with the clarifier a lot so could be the sideband shifting.

Are you running a sideband CB? Or a 10m Amateur unit?

Doppler from an oscillating antenna would be at the rate the antenna is waving, and I doubt you could keep up with a "clarifier"/"delta tune"/RIT knob.

If it is a different "clarifier" setting depending upon who you are hearing it might be doppler (though I wouldn't expect much from two vehicles moving towards or away from each other)... Skip/ionosphere effects maybe...

(I have an unused Cobra 29 still in box somewhere -- too big for mobile mounting these days, unlike the days when one could fit a Kenwood TS-5xx in the gap under the dash and the transmission hump; forget if the 29 is labeled clarifier or delta tune, delta tune was more meaningful on AM rigs)

 

[kidphc]

Are you running a sideband CB? Or a 10m Amateur unit?
Doppler from an oscillating antenna would be at the rate the antenna is waving, and I doubt you could keep up with a "clarifier"/"delta tune"/RIT knob.
If it is a different "clarifier" setting depending upon who you are hearing it might be doppler (though I wouldn't expect much from two vehicles moving towards or away from each other)... Skip/ionosphere effects maybe...
(I have an unused Cobra 29 still in box somewhere -- too big for mobile mounting these days, unlike the days when one could fit a Kenwood TS-5xx in the gap under the dash and the transmission hump; forget if the 29 is labeled clarifier or delta tune, delta tune was more meaningful on AM rigs)
 

I was using an anytone at6666 10m unlocked so I could listen onto 11m ssb. Frankly, if I remeber I was experiencing the same thing sitting still. So can't tell if it was people just off frequency. But it was a lot of playing with the clarifier to get it to sound natural or atleast the way it was suppose to be.

Not like that unit has the best clarifier. Hell even find neutral on it was spin all the one way then other and try to find the middle. Kinda wish it had a center detent. Crap I wish all of the controls gain, power and squelch are all had detents. Far cry from the Ft991a at home.

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[kidphc]

So some reporting to do from real world, only about a week or so.

Repeater is on top a water tower (about 100' up) at about 427 feet in elevation for the base of the watertower. 

Comet 2x24sr (hood shoulder mount) Using the FTM400XDR

• The Comet 2x24sr I received a lot of reports of breaking up, barely getting through the noise floor (40 watts), in/out with receptions. and a lot of dead spots when connected to a very good repeater.
• Dead spots, lots of static
• Was able to test with a repeater in Glenmount MD, to Frederick, MD. Got about 30.36 miles (48.4 km) of rolling hills, elevation changes sometimes up to 100 feet. Spotty, but was making it in with 15watts hashing getting bad.

1/4 Laird Unity antenna: (on roof 18" in front of rear hatch lip) Using a CDM1250

• Decent, very small, tip barely clears the roof rack. Much better reception and transmission when compared to a shoulder/hood mounted Comet 2x24SR antenna.
• reports of a lot of scratchiness and but legible.

Laird B4505CNS 5/8 over 5/8 No Ground Plane. (on roof 18" in front of rear hatch lip) Using a CDM1250

• So far my favorite. Until i start clipping garages and such.
• Best reports (Sound like you are sitting next to me.)
• Little to no scratchiness even at low power
• Area I call the Mormon Temple Curse, is a dip and snakes around the Temple. It is a bad dead spot for this repeater. The highway dips there and gets curve. I call it the curse, since it is where the highway snakes with high angle banks, sun peers through 3/4 of the year with the visual attraction of the temple.
  • It is the only antenna, that makes reliable contact transmission/reception with a little scratchiness on receive and transmission. (a lot of it is the antenna location on the truck)
• Now i get occasional squelch breaks, and transmission from other repeaters (Towson and Ellicot city) that  between 18-26 miles away depending on my local topography.

 

 

 

T

 

[marcspaz]

12 minutes ago, kidphc said:

The Comet 2x24sr I received a lot of reports of breaking up, barely getting through the noise floor (40 watts), in/out with receptions. and a lot of dead spots when connected to a very good repeater.

 

 

I was using the Comet 2x24sr for about a year.  I am not surprised to hear your feedback.  The antenna sucks.  It's a compromise on all bands and wasn't designed to perform well on the 467 MHz range.  Comet say the top-end in 465 MHz.  Its an 'ok' antenna for close range split duty between Ham and FRS/GMRS if you are actually doing some type of SAR/USAR mission in close proximity on low power, but anything over 2 miles and I just can't get good performance out of it. 

 

And I think it's the design, as 462 MHz is at the top of the design scope and 467 MHz isn't even a part of it.  I started off with the NMO version.  I ended up getting a second one in a UHF mount and it had the same issues. 

[kidphc]

3 minutes ago, marcspaz said:

 

 

I was using the Comet 2x24sr for about a year.  I am not surprised to hear your feedback.  The antenna sucks.  It's a compromise on all bands and wasn't designed to perform well on the 467 MHz range.  Comet say the top-end in 465 MHz.  Its an 'ok' antenna for close range split duty between Ham and FRS/GMRS if you are actually doing some type of SAR/USAR mission in close proximity on low power, but anything over 2 miles and I just can't get good performance out of it. 

 

And I think it's the design, as 462 MHz is at the top of the design scope and 467 MHz isn't even a part of it.  I started off with the NMO version.  I ended up getting a second one in a UHF mount and it had the same issues. 

2x24sr definitely is a very big compromise antenna. I think At mid GMRS Band around 465, it was about 2.1 SWR. The Laird 5/8 over 5/8 hasn't been cut, but by the factory cut chart 472 is right around 1.75 in its stock form. Still waiting on adapters, will take measurements later.  The Laird completely kills the Comet in everything from weight, construction, and rigidity. If you have held a 2x24 you know it is heavy antenna that feels unbalanced(like it is a sword or something) when in the hand, the Laird feels about 3x heavier.  I might get rid of the 2x24sr, because good grief the antenna is so stiff the mount is always shimmying.

Yes the Laird 5/8 is 2x-3x the cost of the Tram or its competitors. I feel it is justified. Plus the famous Laird center contact pin, no more bending tabs. Even the center pin on the Laird makes the Comet push center pin, feel like a chintzy toy out of a gumball machine.

[WRKC935]

Ahh, go read this

https://en.wikipedia.org/wiki/Rayleigh_fading

Explains a LOT of what you are experiencing. 

 

[KAF6045]

1 hour ago, WRKC935 said:

Ahh, go read this

https://en.wikipedia.org/wiki/Rayleigh_fading

Explains a LOT of what you are experiencing. 

 

Or the related article https://en.wikipedia.org/wiki/Rician_fading

Quote

Rician fading occurs when one of the paths, typically a line of sight signal or some strong reflection signals, is much stronger than the others

Rayleigh seem more dependent upon a very crowded environment (they studied Manhattan, but maybe like my house? I can walk around with a radio tuned to a local low-power FM broadcast and find dead spots every couple of feet -- and even NOAA weather channel shows 6ft radius circular reception spots in my driveway)

[WRKC935]

8 minutes ago, KAF6045 said:

Or the related article https://en.wikipedia.org/wiki/Rician_fading

Rayleigh seem more dependent upon a very crowded environment (they studied Manhattan, but maybe like my house? I can walk around with a radio tuned to a local low-power FM broadcast and find dead spots every couple of feet -- and even NOAA weather channel shows 6ft radius circular reception spots in my driveway)

Anything over 1/4 wave in length will reflect RF.  Doesn't need to be anything real substantial.  Because of what's required, the issues begin to manifest in the higher frequency spectrum, because more things are at least that size.  While an apartment in Manhattan would certainly have this issue, it does show up in area's that are NOT nearly as congested as that.  Road signs, other vehicles, metal poles, all are over 6 inches and therefore can create multipath.  And the other part of it is the timing and wavelength for how out of phase a signal has to be.  For AM radio, very low frequency, if this was even a thing the dead spots would be hundreds of feet in diameter, buy would require numerous HUGE objects to create the problem.  When wavelength is much shorter, then the area's of phase attenuation due to multipath are much smaller, again typically within one wavelength of the signal you are receiving. 

Been through A LOT of this with simulcast radio systems where multiple transmitters were on the same frequency and their reference oscillators were referenced to GPS.  Audio launch and PL launch times are also down to the millisecond to keep things working.  With the systems I work on I am actually able to adjust the overlap area's of issue and put them in fields and other non-occupied locations and away from places that police and fire personal would need to use radios.

 

[kidphc]

Ahh, go read this
https://en.wikipedia.org/wiki/Rayleigh_fading
Explains a LOT of what you are experiencing. 
 

Intresting read.

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[Lscott]

9 hours ago, KAF6045 said:

Or the related article https://en.wikipedia.org/wiki/Rician_fading

Rayleigh seem more dependent upon a very crowded environment (they studied Manhattan, but maybe like my house? I can walk around with a radio tuned to a local low-power FM broadcast and find dead spots every couple of feet -- and even NOAA weather channel shows 6ft radius circular reception spots in my driveway)

That likely has more to do with diffraction effects. I've experienced that at audio frequency where I work. When somebody was running a 1/2 hour short circuit test, on an inverter setup for 10KHz, just walking a dozen feet one way or the other the high pitch screaming fades out noticeably, and there was nothing in the way.