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Total posts : 45366Since impedance is an across variable it is necessary to specify between which two points it is measured. In other words, is there a conductor going to ground from the 3 meter elevated feed point?
Neil
Total posts : 45366[quote=radio8z]Since impedance is an across variable it is necessary to specify between which two points it is measured. In other words, is there a conductor going to ground from the 3 meter elevated feed point?
Neil
[/quote]Let’s assume there is not.
Total posts : 45366OK. Suppose I have an impedance bridge or meter. I connect one lead to the antenna. Where do I connect the other lead?
Neil
Total posts : 45366[quote=radio8z]OK. Suppose I have an impedance bridge or meter. I connect one lead to the antenna. Where do I connect the other lead?
Neil
[/quote]Ok. How about 3 meters downward….
Total posts : 45366Then that lead would be the conductor connected to ground or not that I previously mentioned. In either case it would radiate and thus affect the impedance at the feedpoint.
* John, I just edited this post to explain further my point. Your question is a good one and I would be interested in the answer. My point was that I needed a description of the model in order to do a calculation. As I understand it, the model is a CB whip 3 m. above ground and the feedpoint is between the antenna base and a conductor going to ground. Since practical grounds have resistance, should this be included? I am new at easy NEC but I will give it a try. Maybe someone who is more talented with NEC will beat me to it. *
Neil
Total posts : 45366Well here’s what I get from EZNEC.
Antenna: 102″ (CB whip)
Antenna diameter: .2″
Elevation to base of antenna: 3 meters.
Source: 3 meters (base of cb whip)
Load: 3 meters (base of cb whip)
Length of ground lead to ground: 3 meters.Results: minimum SWR with j=3308 (capacitive reactance of antenna equivalent to C=29.9pf.
The type and quality of the ground only changes the R part of the antenna reactance, not the J part. The better the ground, the lower the R part of the reactance. This demonstrates that when you cancel the J part with a loading coil of equivalent and opposite J, you are left with just the R part. The better the ground the lower the R.
EZNEC allows you to play around with the ground quality. This doesn’t change the antenna J part of the reactance. However, the better the ground, the less loss you will have. Also, your transmitter should be optimized to drive less than 50 ohms if you have a very good ground (maybe in the range of 10 to 20 ohms).
Phil B
Total posts : 45366Thanks Phil, you beat me to it and that is fine. I am really new with this program and don’t trust my results.
As expected according to your report the capacitive reactance dominates and the R component is, as you said, dependent on the ground.
I am interested in the reference to minimum SWR since an antenna by itself has no defined SWR. Unless the antenna is considered as part of a system it seems there is no SWR to minimize.
Neil
Total posts : 45366[quote=radio8z]I am interested in the reference to minimum SWR since an antenna by itself has no defined SWR. Unless the antenna is considered as part of a system it seems there is no SWR to minimize.
Neil
[/quote]Hi Niel,
Yep, I know what you mean. I just used the SWR display to view the resonant dip at the operating frequency when the J part of the source impedance is equal (and opposite) to the J part of the load impedance. The SWR display is also useful to illustrate the effect of a mismatch between the R part of the source and load impedances at resonance.
Phil
Total posts : 45366Phil,
Thanks for the explanation. As I said, I have not had much experience with the EZNEC program. No need to answer since I will find out for myself but I presume you set the source Z and the program plots the SWR. That makes sense.
Neil
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