Explanation of this video.

[WRKC935]

Here's the video.  THis was posted on Facebook in the group.

So some explanation.

Think of an antenna's radiation pattern as a donut looking at it from the side.  It looks a lot like to circles in side by side with the antenna in the middle between the two circles.  This is the pattern of a UNITY (0 dB) gain antenna.  Something known as an isotropic radiator.  When you look at antenna gain numbers they will typically be listed in dBi.  The i in there refers to this type of antenna.  But the important thing here is the antenna radiates power in ALL directions almost equally with a null directly above and below it. 

So then we start talking about antenna GAIN.  Gain manifests in a manner similar to crushing the donut equal distances from the bottom and the top.  With the pattern being pushed out to the horizon at the cost of radiation both above and below the plain that the antenna is on.  If you take this to extreme you CAN in some situations create a null at ground level under the antenna and out some distance from the tower.  BUT you need enough height to make this happen.  I actually know of a repeater that has this issue that is currently on the air at a local TV station.  The repeater and antenna are located at 750 feet above the ground and the antenna is a DB-420 high gain UHF antenna.  This repeater will talk to Indiana just fine, but you can't talk to it unless you are a minimum of 5 miles from it from the ground.  The helicopter that it was setup to communicate with had no issues because it flew in the pattern of the antenna. 

So how does this effect your GMRS repeater and it's coverage??? It DON'T.  Not unless you happen to have a VERY high gain antenna that is on a very tall tower or happen to be near the top of a high hill above a valley that is 700 foot down and inhabited with people you want to talk to.  THEN and ONLY THEN does this come into play.  

I have a DB-420 antenna mounted at 180 feet up at the tallest point in the county.  I have no issues with using it to communicate with locally and even from the tower lot.  It works fine. 

Another thing you need to consider if you are looking at down tilt of an antenna.  That being the UP tilt of the opposite side of the antenna.  If you were to tilt your antenna DOWN to talk into a town or someplace below your antenna, then you tilt the other side of the pattern UP into space killing the ability to communicate in that direction at all.  And frankly there are better ways of dealing with an antenna system that has this issue with local coverage.  You add a second antenna and a power divider lower on the tower and if needed a yagi or corner reflector antenna to cover the area you need covered and only having a SMALL effect on the overall coverage footprint form the main antenna. 

[kidphc]

On 9/18/2022 at 11:25 PM, WRKC935 said:

Here's the video.  THis was posted on Facebook in the group.

So some explanation.

I have a DB-420 antenna mounted at 180 feet up at the tallest point in the county.  I have no issues with using it to communicate with locally and even from the tower lot.  It works fine. 

Another thing you need to consider if you are looking at down tilt of an antenna.  That being the UP tilt of the opposite side of the antenna.  If you were to tilt your antenna DOWN to talk into a town or someplace below your antenna, then you tilt the other side of the pattern UP into space killing the ability to communicate in that direction at all.  And frankly there are better ways of dealing with an antenna system that has this issue with local coverage.  You add a second antenna and a power divider lower on the tower and if needed a yagi or corner reflector antenna to cover the area you need covered and only having a SMALL effect on the overall coverage footprint form the main antenna. 

I agree with your synopsis. Nice setup btw. I can only think of like 1 or 2 instance where this would even be useful. Even in those instance I would say a director of some sort or yagi beam antenna would perform better. Most repeater owners don't want the expense of a repeater to have a concentrated area of reception, again most. I would think most want the largest foot print around the antenna, really comes down to the purpose.

I have always kinda wondered about the wagging of my antennas on my truck and it's effect on transmission and reception. But haven't noted any really doppler or picket fencing. Biggest changes always came down to my ground plane on my truck. Mount it on the fender, meh... up on the roof of the truck and it started competing with some base stations.

[marcspaz]

 

I could be mistaken, but the person who made that video doesn't seem to understand far-field antenna radiation maps.  Not to mention, if you are like me and have a nice tall high-gain antenna on the roof or on a tower, when the wind blows, the antenna is going to sway around a heck of a lot more than factions of a degree... in many cases, several degrees.

 

  

48 minutes ago, kidphc said:

I have always kinda wondered about the wagging of my antennas on my truck and it's effect on transmission and reception.

 

It does make a difference, but mostly on the fringe of your max range.  The longer the distance, the more it makes a difference. 

 

I really experience this with my mobile HF radio when I am talking several thousand miles.  if I am driving, I may be able to hear someone, but they can't hear me because I'm driving 60 mph and my 86"+ antenna is waving all over the place... impacting the take-off angle.  If I pull over and point the truck in the direction of the station I want to talk to (my antenna is in the back of the truck, creating a somewhat directional dipole with the whip and body), I go from the distant station not hearing me at all, to getting a perfect signal report.

[WRKC935]

8 hours ago, kidphc said:

I agree with your synopsis. Nice setup btw. I can only think of like 1 or 2 instance where this would even be useful. Even in those instance I would say a director of some sort or yagi beam antenna would perform better. Most repeater owners don't want the expense of a repeater to have a concentrated area of reception, again most. I would think most want the largest foot print around the antenna, really comes down to the purpose.

I have always kinda wondered about the wagging of my antennas on my truck and it's effect on transmission and reception. But haven't noted any really doppler or picket fencing. Biggest changes always came down to my ground plane on my truck. Mount it on the fender, meh... up on the roof of the truck and it started competing with some base stations.

Yes, the applications are LIMITED for enhancing coverage through down tilt antenna's or fill antenna's for specific area's.  It's mostly used in mountainous regions that have towers with a TON of HAAT (height above average terrain).  If your repeater happens to be on a mountain top and it's the tallest around, and PART of teh area of interest is in the valley below then this type of stuff comes into play.  But if you are JUST trying to serve the town at the bottom of the valley, then you put the antenna in the bottom of the valley somewhere on a tall structure and be done with it. 

I have heard of guys sticking MOSTLY ham antenna's way the hell up in the air on a tall tower on a tall mountain and can't understand why the coverage sucks.  And what's funnier about that is the area that actually gets the best coverage is without any residents.  It's forest, or wheat fields but NO PEOPLE. There was a time that even the commercial radio guys put the biggest antenna on the tallest tower if you want it to talk.  And that is rarely the best option anywhere.  300 feet is about the max if you are in a flat area.  And hilly terrain, you really need multiple sites with multiple repeaters to cover that topography.  Nothing else really works.  You either end up covering the hill tops, or you have a small footprint of coverage.