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Directivity from multiple base antennas?

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Bluerunner
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Directivity from multiple base antennas?

Post by Bluerunner » Saturday, 24 December 2016, 9:32 AM

Like many I have three (sometimes more) base station antennas, one starduster, one half wave horizontal dipole and a end fed vertical sleeve. They are all about the same height and spaced from 20 to 35 feet apart.
I have often wondered if the unused antennas could be acting like directors or reflectors to whatever antenna is active.
I don't have enough locals around me to make test at significant distances and skip is unreliable.

I am wondering if there is a way to neutralize the unused antennas to cancel any resonance and eliminate the possibility of unwanted radiation pattern modification of the active antenna.
It seems like this may be a common enough concern that some ham may have studied the problem (if any) and made some technical determinations.
Any thoughts will be appreciated.




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Re: Directivity from multiple base antennas?

Post by 443 Arizona » Saturday, 24 December 2016, 12:23 PM

so hook some other radios to the idle ant's and see what you are getting for feedback.(of course your going to blow your own ears out so just leave them off and see what feedback still comes thru.) that will be a good clue as to what they act like.
you might want to get creative and put a voltage to the idle set of ant's and get them to resonating with your active ant.

the answers to this ant.scenario," is like a box of chocolates,,,,,,", :dwarf:



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Re: Directivity from multiple base antennas?

Post by 231 » Saturday, 24 December 2016, 18:26 PM

There is always interaction of some kind...even when we think there isn't. We merely try to reduce interaction by spacing (hopefully in non-resonant lengths).

It might be possible to ground both the center lead & shield of the feed line to the antennas you don't want to use. I'd think it would turn even the resonant part of the antenna(s) to become less 'interactive' to some extent. But truly the only way to totally eliminate 'stuff interacting' is to remove it/them entirely. In most cases I seriously doubt you'd see a significant difference...certainly not enough to warrant not having it available instead. Utilizing an Alpha Delta switch box (which is suppose to ground center leads when not in use) might be an option as well. They are rated at 2-3kw if memory serves and a popular switch box among hams. I use a remote switch box that takes non-used relays to ground (model 519R 9 position). I've never cared about interaction on my set up...I have currently 4 antennas for different bands up.

I doubt interaction is significant enough to notice much on your set up...especially if they are mounted lower to the ground (under 40' to the feed point) or close to roofs, etc. Just my thoughts.

Good luck.



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Re: Directivity from multiple base antennas?

Post by MDYoungblood » Saturday, 24 December 2016, 21:41 PM

Like what 231 says, I ground the unused antennas. With the distance of 20 to 35 feet I doubt if there would be any signal deviation and if there is it probably is enhancing the signal in that direction. The only thing I've read in the ham forums pertain to stacking antennas.

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Re: Directivity from multiple base antennas?

Post by The DB » Sunday, 25 December 2016, 11:43 AM

There are really two things that matters, distance and polarization.

With nearby antennas, they will interact with each other. There is simply no way around it, and distance helps. The key element for distance is the further away the antennas are the less they will interact. For an example of a set of "antennas" that will react in such a way that a benefit is provided, look at Yagi antennas, unfortunately, the beneficial benefits are few and far between, you are unlikely to find such a situation when it comes to mounting antennas where you can get them, which is why distance helps.

The other big thing is polarization. Horizontal and Vertical polarizations simply don't interact well with each other. In theory, there is no interaction, but in reality there is a small amount, but only a very small amount. If you have two antennas, just having one vertically polarized and the other horizontally polarized is like adding quite a bit of distance between them.


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Re: Directivity from multiple base antennas?

Post by Bluerunner » Sunday, 25 December 2016, 23:10 PM

I wonder...

If I put a power meter on the unused antenna cable "backwards", terminated in a dummy load, would it indicate an appreciable amount of power consumed from the nearby active antenna?

Maybe a 5 watt power meter measuring the field absorbed from the active antenna 20-30 ft away energized with 100 watts (or more). I know guys have burned out radios on adjacent antennas so there should be a measurable amount of power there.

It seems to me that measuring the power consumed would be equivalent to the strength of the reflected or directed signal that would be radiated from that parasitically energized antenna.
I know the distance affects the phasing so the pattern may or not be significantly affected. The tuning (as a director or reflector) of the parasitic antenna may also be effected by the random coax length attached to it (I can't think of the name of the vintage CB antenna that switched different coax lengths to change the element tuning as an example).

I've seen tables & calculators that give info on parasitic element spacing & tuning and the calculated beam-width and gain, even to extreme dimensions. However the profound variability of elements with shorted or unterminated, or dummy load terminated feed-lines attached to them of random lengths will not easily enter into any calculations.

I can switch between my three antennas while reading a skip transmission and often get inconsistently different readings. One station my be equivalent on all three antennas, another station my boom in on one antenna and be low or missing from one or both of the other two. (when I happen to have three vertical antennas up). I guess it can but I wonder how a distance of 20-30 feet can yield such different results.

I guess I'm thinking that a simple reading of absorbed power may be useful as an easy estimation of the significance of the probability of radiation pattern skewing, even if it cannot determine what the skew actually is.

Maybe I'm asking the wrong questions, probably it is insignificant, but maybe not. As much as many of us agonize over maximizing every aspect of our operation, I can't help but feel that someone has gone down this path before and generated reams of data and come up with a quantifiable measure and the practical means to obtain it.

As soon as I get my 5 watt power meter back I will try it out. I'm confident I will get a reading, but what to do with it, what will it tell me?

It will probably tell me to go get another beer and cigarette and stop worrying about insignificant issues.



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Re: Directivity from multiple base antennas?

Post by 443 Arizona » Monday, 26 December 2016, 1:47 AM

there you have the idea Mike, put the FS meter in play on the other ants.
i guess thats what i was trying to say previously by using other radios.
if those ant's become resonant , then determine if you can use them as directional gain or reflection,
are they on the same height, level,polarization,etc.
i think grounding them would only create a deadzone/ juice-sucking node.
apply a 9volt battery to the centerpin and get them "live" and maybe aid in directivity :idea: or put a capacitor on to absorb some microvolts and maybe act as a trimmer on the undriven ant.
(clip it right to the ant.),,or ....hook the ground from the driven ant. to the radiating element of the reflecting ant.
heck i dont know at this point, :confused:to many ideas :dwarf:
too much eggnog. :compress: or not enough :geek: :drunken: :albino: :icon_e_surprised: :icon_rolleyes: :lol: Merry Christmas



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Re: Directivity from multiple base antennas?

Post by Slim Pickins » Monday, 26 December 2016, 6:59 AM

If you are interested in gain from multiple antennas and have a few verticals available why not look up vertical phased array antennas or 4 square antennas. Remember that CB antenna from years ago that used 3 vertical dipoles in a triangle configuration? The 2 passive elements acted as reflectors if memory serves correctly. I'm not sure what the gain would be on phasing multiple verticals on 11m but it might be a fun experiment. The 4 square set-up is normally used on the lower HF ham bands where a beam would be entirely impractical. Imagine a 4 element 80m beam and all the associated hardware you would need :shock: At any rate having 4 verticals in a fixed pattern with proper spacing and the ability to select the driven antenna should give you selectable directivity without needing a rotator. As to what the gain figures and nulls would look like? Experimentation would tell.


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Re: Directivity from multiple base antennas?

Post by The DB » Monday, 26 December 2016, 10:19 AM

Bluerunner wrote:If I put a power meter on the unused antenna cable "backwards", terminated in a dummy load, would it indicate an appreciable amount of power consumed from the nearby active antenna?
Think field strength meter, and you don't have to hook the power meter up backwards, unless it is a directional meter, which almost none used in CB are directional.
Bluerunner wrote:Maybe a 5 watt power meter measuring the field absorbed from the active antenna 20-30 ft away energized with 100 watts (or more). I know guys have burned out radios on adjacent antennas so there should be a measurable amount of power there.
I doubt it will even move the needle on the meter of a 5 watt power meter enough that you would see it. Most field strength meters, which pick up a signal like the antenna you describe above, work in the milliwatt and microwatt regions. Watts is simply to much power.
Bluerunner wrote:It seems to me that measuring the power consumed would be equivalent to the strength of the reflected or directed signal that would be radiated from that parasitically energized antenna.
I know the distance affects the phasing so the pattern may or not be significantly affected. The tuning (as a director or reflector) of the parasitic antenna may also be effected by the random coax length attached to it (I can't think of the name of the vintage CB antenna that switched different coax lengths to change the element tuning as an example).
If coax length affects the tune of an antenna you have a problem. When that happens the coax itself is actually acting like part of the antenna. You aren't tuning the antenna, you are just manipulating the readings that your SWR meter picks up.
Bluerunner wrote:I can switch between my three antennas while reading a skip transmission and often get inconsistently different readings. One station my be equivalent on all three antennas, another station my boom in on one antenna and be low or missing from one or both of the other two. (when I happen to have three vertical antennas up). I guess it can but I wonder how a distance of 20-30 feet can yield such different results.
Because of the difference in a vertical and horizontal polarizations it is often suggested to have one of each when you have two antennas up.

The difference you are seeing with the vertical antennas would have to be a difference in their radiation patterns. If antenna one has a null in the same direction that antenna two has a peak, and that is the direction the signal came in on, antenna two will have a much stronger signal strength. If another signal comes in in the direction of a peak on antenna one the reverse would be true. This is just as true with both the horizontal and vertical directions. A lot of people default to trying to get their "radiation angle" as low as possible but their are trade offs to doing that as well. A low radiation angle will not help you if it moves a null in you pattern in the direction that skip is sending signals your way.
Bluerunner wrote:I guess I'm thinking that a simple reading of absorbed power may be useful as an easy estimation of the significance of the probability of radiation pattern skewing, even if it cannot determine what the skew actually is.
Short of modeling, it is nearly impossible to determines the skew of antennas. Get this, there are people out there that can actually use multiple elements like that and control the skew, although they generally use multiple antennas that are all fed with certain phasing. The further the second antenna is away from the first, the less of an effect. It would be interesting to determine how far apart the parasitic element would have to be away from the antenna to have no more than an insignificant effect. At first I would say somewhere between one and two wavelengths.
Bluerunner wrote:Maybe I'm asking the wrong questions, probably it is insignificant, but maybe not. As much as many of us agonize over maximizing every aspect of our operation, I can't help but feel that someone has gone down this path before and generated reams of data and come up with a quantifiable measure and the practical means to obtain it.
Not that I have seen, and I am very well read when it comes to antennas. I could perhaps do some modeling for you if you wish, modeling is where this data would have come from anyway.
Bluerunner wrote:As soon as I get my 5 watt power meter back I will try it out. I'm confident I will get a reading, but what to do with it, what will it tell me?
Unfortunately, not much, if you get a reading at all. Go ahead and do it though, and report on your result.
Bluerunner wrote:It will probably tell me to go get another beer and cigarette and stop worrying about insignificant issues.
This is something that many people never learn. It takes quite a bit to make a noticeable change in an antenna system, much more than many people think. A lot of people will make a bunch of tiny changes that add up to next to nothing. Antennas don't have to be perfect to work well, they just need to be close.

I made a quick pattern comparison for you.

Image

The blue line is a single antenna and here as a reference, the red line is two antennas 20 feet apart, and the green line is two antennas 30 feet apart.


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Re: Directivity from multiple base antennas?

Post by Bluerunner » Monday, 26 December 2016, 13:05 PM

Thanks for the model DB. It shows something is going on theoretically anyway.
One of those shorting antenna switches my help avoid some problems too. I'll put one on my Christmas list.
A FS meter just gets pegged and I would have to use an attenuator of some sort. Just figured a power meter might be in the range. Easy enough to find out.

That CB antenna I referenced is a "Super Scanner". I saw a vertical wire one for low band use in one of the antenna books. Just thought it interesting how the coax could change the tuning and foul up estimations.

Thanks for the comments. Always enlightening and interesting.




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