Actually, you have to do both. A real implementation will have an
attached transmission line. This transmission line, at a particular
frequency, may appear as an open, a short, or most anything in between,
depending on the length of the line, and how the unused end is
termninated. So, you are probably OK if there is no interaction with the
source either open or shorted.
You can also use this effect to good advantage. You can reduce antenna
interaction by detuning the unused antenna, by proper choice of length
and termination of the unused line. Works at only one frequency,
however. (unless a switching arrangement is used).
Note: Another NEC2 quirk to watch out for: You can't place a load
directly across a source by placing it at the same location. It usually
places the load in series instead of parallel, and sometimes it does
something different (never bothered finding out exactly what, just that
it doesn't work correctly). Don't know about NEC4.
Jerry, K4SAV
Eric Scace K3NA wrote:
>Hi Dick --
>
> Some thoughts:
>
>1. Simply deleting the source from the unused antenna changes that
>element from a dipole to a continuous piece of metal -- which is NOT
>what you have.
>
> Rather than deleting the source, you must leave the source present.
>
> Now you have to decide what impedance the unused piece of coax
>places across the drive point of that antenna at the model frequency.
>(A piece of coax running into the shack, not connected to any
>transmitter or receiver, presents some impedance across the drive point
>of the antenna.) Since this is not an actual system you can measure yet,
>you don't know what this impedance will be. That may be an issue for
>some lengths of coax. And it may be different when the antenna is
>unused vs. connected to a receiver (i.e., terminated in a nominal 50 +
>j0 ohm).
>
> For playing around, you could assume no transmission line is
>connected to the unused antenna. In this case you can specify that NO
>current is applied to the unused antenna's drivepoint.
>
> However, beware of an odd twist in NEC code. If you specify ZERO
>drive current (or voltage), NEC assumes the drive current should be 1A
>at 0°! (The explanation for this is too tedious to bother with in a
>short note).
>
> So, the correct way to specify zero current (or voltage) is to
>actually specify a very small value. I have found that NEC4 will accept
>1e-10 (e.g., 100 nanoamps) is about the smallest value which the
>calculating engine will accept. I have not tested NEC2 engines in this
>regard.
>
> Remember, in a real implementation, the feedline (with its balun)
>will put a parallel impedance on the unused driven element that will
>have some impact on the response of the unused antenna. A proper study
>will need to take this into account.
>
>2. I assume you are looking at two slices through the driven antenna's
>pattern: an azimuth slice at some important elevation angle, and an
>elevation slice through the main beam.
>
> In severe interactions, you will see a change in the main beam's
>gain or pattern shape. This disruption will clearly show up in a change
>of impedance for the driven antenna.
>
> But many interactions will not show up as a significant change in
>driven antenna feedpoint impedance. Instead, the main beam changes
>slightly (perhaps a dB) and the lost power shows up in new minor lobes.
> These minor lobes can occur anywhere -- quite possible in directions
>that you do not see in those two slices through the antenna's pattern.
>Those minor lobes open up the antenna to receiving more QRN and QRM from
>directions which are not of interest to the operator. To find these, a
>more comprehensive plot of the entire 3-D antenna pattern must be
>examined. One way to do this has been outlined in my article series in
>the National Contest Journal (parts of which are posted on the NCJ website).
>
>73,
> -- Eric
>
>on 05 Jun 21 15:27 Dick Green WC1M said the following:
>
>
>>Hi. I'm trying to use EZNEC 4.0 to determine whether there would be any
>>interaction between a 2-el 40m beam at the top of a tower and various beams
>>placed lower on the tower. For example, I've tried putting a 40-2CD at 105'
>>over a 3-stack of 4-el 15m monobanders at 90'/60'/30'. The model showed no
>>significant difference in the 15m pattern with or without the 40m beam, and
>>no significant difference in the 40m pattern with or without the stack. Same
>>story for 3-stacks on 20m and 10m at the same heights. I even tried putting
>>a 5-el 15m beam at 100', only 5 feet from the 40m beam, and got no changes
>>in pattern for either antenna. Seems to me that there would be some
>>interaction between 40m and 15m antennas spaced that close. Is that
>>incorrect, or is there a problem with my model?
>>
>>FYI, I'm deleting the source for the antenna that's not on the band in
>>question. In other words, if I'm checking the pattern for the 15m stack, I
>>leave its sources intact and either delete the source for the 40m beam or
>>set the V to 0 (results are the same either way.) If I'm checking the
>>pattern for 40m, I leave its source intact and delete the sources for the
>>3-stack. Is this the right way to do it? The only difference from reality is
>>that I'm not including booms and transmission lines for any of the antennas.
>>
>>If it appears to the experts that I'm doing things correctly, can anyone
>>give me a good example of two antennas that should show some interaction
>>when placed in close proximity? I'd like to model a case of known
>>interaction so I can make sure I'm doing this right.
>>
>>73, Dick WC1M
>>
>>
>
>_______________________________________________
>
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
>questions and ask for Sherman, W2FLA.
>
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>
>
>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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