Unexpected Counterpoise Results...more is not always better
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- Опубліковано 14 жов 2024
- In this video we show how ground conditions can change even at a fixed location. This can affect the effectiveness of your counterpoise. The set-ups discussed are for portable vertical antennas suitable for POTA or emergency use.
Here is a link to a video showing how different counterpoise methods work.
• Ground Vs Counterpoise...
You need to watch the linked video to fully understand what is discussed in this video.
I don't think it is meaninful to try to evaluate the performance of an antenna by reference to the SWR alone. What you are interested in, is the radiation efficiency. The reason we add a counterpoise to the antenna is to increase the radiated field. It can be expected that adding more ground screen area to the vertical antenna system may cause the SWR to increase. If you had a 1/4 wave vertical, for instance, the radiation impedance is about 36 ohms, placing it over typical ground would have a ground resistance of about 16 ohms. These resistances appear in series, so you are burning up a good portion of your RF in the 16 ohms of ground resistance, but the SWR would be close to 1:1 because the total resistance seen by the transmitter is 52 ohms. As you improve the ground screen by adding radials, the ground losses will decrease, but the SWR will go up. If you continue to adjust the tuning of the antenna to provide an optimum match to the transmitter, the radiated field will be increased in spite of the fact the SWR is increasing. It is the field intensity we care about, not the SWR. Yes, many times, the SWR will improve as the field intensity goes up, but that is not necessarily the case and not likely in the case of ground-mounted verticals. A good reference is "Another Look At Reflections" by M. W. Maxwell, also "The Ground Mounted Short Vertical" by Jerry Sevick. A useful practical appreciation of antenna radiation and the function of counterpoises is in U.S. Army Technical Manual TM-11-666, which can be found as a download on the internet.
Sevick's paper is good stuff.
Absolutely correct, low swr does not mean greater efficiency, it only means a better impendence match to the transmitter.
This comment is 100% correct
At some point the ground plane size and its correlation to ground losses is affecting the return loss which seems to peak at a some magic point of correlation between all these variables. The greater the return losses the more chances of power being radiated.
Get them every time. It's like saying a dummy load makes a great antenna. SWR is important but is not everything.
I learned about Faraday Fabric thanks, first time for me.
This video really resonated with me (pun intended) as I've noted the (strong) correlation between soil moisture and the performance of my ground-mounted trap vertical antenna.
When conditions are dry, the antenna seems to benefit from additional ground radials and screen (I haven't tried Faraday cloth yet). When the ground is more moist, it doesn't seem to care as much about all my efforts (though my additional counterpoise efforts don't seem to detract).
Of course, there's loads of variables in all this including general band conditions and man-made interference. With the long wavelengths involved with HF, the counterpoise for a vertical extends well past the distance where I have any ability to improve it (i.e. my neighbors, I'm sure, wouldn't be too keen about me installing radials on their property).
I really enjoy your videos -- thank you for making them!
73 from N8ESP
Thanks for your nice comment.
Wow! Very interesting data Jim! Thanks for doing the "grunt work" on this. One observation... where the two graph lines cross is a point of ground square footage that would seem to work with decent SWR for both wet and dry conditions. This analysis is certainly a good base line for further experimentation. Best 73s Dan K1YPB - On the air since 1962
You make a very interesting point. Thanks for your comment.
I assume your antenna is a base loaded 17ft or 18 ft vertical. If so, with a full ground plane, medium soil conductivity and tuned to resonance (at 3.7 mHz), the input impedance is about 8 ohms -- based on an EZNEC model. In other words, with medium soil conductivity, a relatively high SWR should be expected as counterpoise-to-ground coupling is increased (i.e., ground resistive loss decreased), and vice-versa as ground coupling is decreased (i.e., ground resistive loss increased). For poor soil conductivity, resistive ground loss will be high which will increase antenna input impedance and lower SWR. So, for your case, a high SWR indicates good ground conductivity and good counterpoise-to-ground coupling. And as you decrease counterpoise -to-ground coupling, you increase resistive loss and lower SWR.
Thanks for your comment.
Interesting video and results. I am convinced that the screen and radials capacity couple to the ground and the value of capacitance goes up with a larger screen and hence the capacitive reactance goes down with increasing area. That should be a good thing. I wonder if you did any measurements of real R and X values with the different areas. I am wondering if the real R value ( resistance at resonance ) changed with the increases size of the screen? That could maybe explain the SWR going up while the reactance was still going down. (I am going to watch the video again as I failed to note if the resonant frequency changed any). It has been my experience that the real R does change as well as the resonant frequency as I add radials. I have tried using the RBN ( reverse beacon network ) to detect signal strength differences but unless the difference is 10 dB or more the difference is hard to detect due to fading and other propagation changes. I have not yet had a good way to measure small field strength differences. I may try two identical mobile antennas as far apart as I can get them, one with minimum radials and one with increasing number of radials until I either see a difference or run out of wire! A big problem with two antennas connected to a normal coax switch is the grounds side of the coax is always connected together so I would need to physically disconnect the cable or use a DPDT switch. Again coupling between the two ground systems needs to be avoided in a test like that. Keep up the good work. 73, N4DJ
You are correct about the capacitive coupling to real ground. I demonstrate this in the video linked below. Thanks for your comment,
I wonder how different bands affect the optimum square footage of the ground plane fabric.
Hi Jim … Your productions are always interesting and I appreciate you taking the time to share them! Rookie question: Is it possible that the large wrenches that anchored the Faraday cloth might have an effect on SWR? I reckon you would have considered that so I’m primarily interested in why they don’t have an effect. Thanks again and 73.
Don't forget the cloth is a conductor so we have a conductor laying on a conductor which acts as one continuous conductor. I used to use rocks, but sometimes they are hard to find.
Thanks, Jim. I hesitated to use metal weights but I won’t on going.
Excellent information Jim 👍
Thanks for your comment.
Great video. I would have bet the ranch if someone asked whether or not enlarging your magic carpet improved your SWR. I'm going to try this out because now I'm extremely curious. Thanks again for an informative video...73...KN6KNB
Most of the time more is better. That is why I was so surprised at what I found.
What were the measured R and X values for the various counterpoise configurations?
Did you retune the coil after reach change to the counterpoise? 73 DE W8LV Bill
No I did not retune the coil. Apparently the resonant frequency of the coil and whip changed very little. Thanks for you comment.
Did coax have some chock balun? Your coax is also conterpoise if others are too small or missing.
Doesn't appear there is a 1:!. Jim did use one in some of the experiments he did last year.
No, I did not use a choke in either test.
You need to find the resonance frequency and impedance. This is where the reactance is zero and not where SWR is 1:1.
Yeah, more counterpoises not better has been known for some time; diminishing return at some point. Rather than fretting over putting x number of wires out there, just put down one or two and get on the air. We always work stateside and DX using ten watts or less. Keep it simple and spend the time one the air.
Negligible return I believe, not diminishing. Unless you count effort and expense as the variable that subtracts from the return... and that is relative.
Can you repeat without coax or very short?
You might be changing the freq of the ant. A SWR sweep might give a little more insight.
What is your definition of an SWR sweep? I thought I showed you SWR sweeps.
maybe it has to do with shape instead of size?
I had a similar experience, my 1st thought was to put a choke at the base of the antenna. And the problem went away.
Thanks for your comment.
A resonant vertical antenna is not 50 ohms, therefore a theoretical 1:1 match to a 50 ohm transceiver will not produce the most ideal radiation pattern. Your definition of "better" I guess means better SWR only, not better antenna peeformance.
A resonant vertical over a perfect ground has a resistive component of 37 ohms. Most modern radios have an internal tuner that can match up to a 3:1 SWR. Are you saying we should never push that tune button? The purpose of the video is to show that ground conditions can change.
@@Jimscoolstuff No... I am saying the exact opposite. Using lowest SWR to compare antennas or counterpoise systems is misleading and a mistake often made on social media and by ham youtube hobbyists. At 9:10 in your video you assert that from your SWR readings that it might make sense to "make the magic carpet smaller". I'm not sure how reduced ground coupling will improve vertical antenna performance in any scenario, regardless of incremental SWR changes.
Looks like you need to tune your carpet. Wrap it in a dowel and unroll to optimize.
Thanks for your idea.
It's not unexpected. If bad ground is comparable to jumping in a mud and full ground is comparable to jumping on concrete. Then tuned ground would be like jumping on trampoline.
Interesting analogy. Thanks for your comment..
@@Jimscoolstuff Wish I could come up with something that smart :D Its stolen from a video called Understanding RF ground for antennas. I just tweaked it a bit.