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End Fed Matchbox Orientation??
- Thread starter CB590
- Start date
Waiting on rig, matchbox, coax..
I have a 4' high wood surrounding with wood lattice panels (kinda like a patio with no floor). Should I run coax (LMR-240) near top rail of it or bottom edge just above grass/ground??
I have a 4' high wood surrounding with wood lattice panels (kinda like a patio with no floor). Should I run coax (LMR-240) near top rail of it or bottom edge just above grass/ground??
OK ,if the coax is your counterpoise , I's radiating and is as much a part of your ant as the wire end.
The coax becomes your 'counterpoise' if you don't provide a piece of wire to perform that function. Kinda like the "no-ground" mobile CB antennas. Watch out for burned lips if you get too close to the metal grille on your mike. Sounds like a perfect job for the MFJ 'artificial ground' box.
I'm inclined to take W8JI seriously, for the most part. I won't say he knows everything about the subject, but a guy who runs a four-square antenna array on 160 meters clearly has something on the ball.
73
I'm inclined to take W8JI seriously, for the most part. I won't say he knows everything about the subject, but a guy who runs a four-square antenna array on 160 meters clearly has something on the ball.
73
When it comes to controlling common mode currents with a half wavelength antenna, its not rocket science, although it does take a little more work than some other antenna lengths.
I made a few models to show what I am talking about, first the baseline model with no choke.
As you can see, I chose a length of feed line intended to show a worst case scenario, the currents on the feed line are nearly as strong as the currents on the actual antenna.
On the w8ji page linked above, he said:
And this is true, as I will show you here. The blue mark on the following model is an 8000 ohm choke, one of the higher impedance chokes you can make/use these days...
As we can see, it made little difference, so w8ji's statement above is accurate. The question is why did it make such a little difference? The answer is the choke is a current device, putting it in a location that is almost purely voltage, like the area near the feed point of a half wavelength antenna, is an error in and of itself.
My concern with w8ji's statement on said page is that is as far as he went. He never considered other possibilities.
So what if we place a choke at another location on the feed line? Can we get better results? The answer is yes. The key is to put the choke further away from the feed point, in this case I put a 200 ohm choke about 1/4 wavelength down from the feed point. I want to point out that you would need 40 times the impedance of this choke to match the impedance in the choke above.
Putting the choke in this location I have tested this model with other feed line lengths, even mounting the antenna higher, and it made no real difference at all. The lengths that did show some currents below the choke had only enough to tell that they were present. A higher impedance choke, which would actually be used by people, will minimize those currents even further.
Conclusion, when it comes to chokes, location, location, location. A far superior choke in a bad location will be noticeably less effective than a poor but functional choke in a better location.
Hope this helps.
The DB
I made a few models to show what I am talking about, first the baseline model with no choke.
As you can see, I chose a length of feed line intended to show a worst case scenario, the currents on the feed line are nearly as strong as the currents on the actual antenna.
On the w8ji page linked above, he said:
w8ji said:
And this is true, as I will show you here. The blue mark on the following model is an 8000 ohm choke, one of the higher impedance chokes you can make/use these days...
As we can see, it made little difference, so w8ji's statement above is accurate. The question is why did it make such a little difference? The answer is the choke is a current device, putting it in a location that is almost purely voltage, like the area near the feed point of a half wavelength antenna, is an error in and of itself.
My concern with w8ji's statement on said page is that is as far as he went. He never considered other possibilities.
So what if we place a choke at another location on the feed line? Can we get better results? The answer is yes. The key is to put the choke further away from the feed point, in this case I put a 200 ohm choke about 1/4 wavelength down from the feed point. I want to point out that you would need 40 times the impedance of this choke to match the impedance in the choke above.
Putting the choke in this location I have tested this model with other feed line lengths, even mounting the antenna higher, and it made no real difference at all. The lengths that did show some currents below the choke had only enough to tell that they were present. A higher impedance choke, which would actually be used by people, will minimize those currents even further.
Conclusion, when it comes to chokes, location, location, location. A far superior choke in a bad location will be noticeably less effective than a poor but functional choke in a better location.
Hope this helps.
The DB
I ended up with 9' height at feed (matchbox mounting height) tapering up to approx 27' in the tree, making slow left angling down to about 14' (about 9' length).. Using calcs on line I came up with about 39' wire length.. Still waiting on matchbox, etc..
Black line is coax planned layout, yellow dots are antenna.. Deck top rail (wood) is 18' long.
Black line is coax planned layout, yellow dots are antenna.. Deck top rail (wood) is 18' long.
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I made several EFHW 11 meters antennas without counterpoise, but always used, or advised those for whom I made them to use a choke with them. I experimented with radials to check for any noticeable performance differences and used my analyzer to attempt to note matching changes. I don't recall anything noticeable UNTIL I worked painstakingly readjusting the variable cap and inductor tap point of my matcher with a full set of radials around the bottom. I did not see increased db of gain as I have no means of testing any such other than a radio S meter, but it was quieter with a lower noise floor, and responded to fainter rx signals, and I got excellent tx reports. I found it to be a really good antenna rivaling the 5/8^ antennas I had made.
My experience is purely anecdotal, but it represents what my antennas were about - HOMEBREW usabilty. This one was excellent in that respect.
My experience is purely anecdotal, but it represents what my antennas were about - HOMEBREW usabilty. This one was excellent in that respect.
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@The DB
Excellent explanation in a short time. Curiously, In all my doings and reading, this approach to choking a halfwave endfed 1/4^ below the feedpoint (high voltage point) was never presented to me except perhaps with the Astroplane being choked 1/4^ below the ring.
I was even encouraged to place a choke at the coax entry point of a vertical sleeved dipole, the bottom of the lower tube, similar to the GM. (I guess I wasn't looking closely enough.) Isn't this also a high voltage point? Answer in a different thread for the sake of courtesy.
(My 80m doublet is choked at the shack entry point.)
Excellent explanation in a short time. Curiously, In all my doings and reading, this approach to choking a halfwave endfed 1/4^ below the feedpoint (high voltage point) was never presented to me except perhaps with the Astroplane being choked 1/4^ below the ring.
I was even encouraged to place a choke at the coax entry point of a vertical sleeved dipole, the bottom of the lower tube, similar to the GM. (I guess I wasn't looking closely enough.) Isn't this also a high voltage point? Answer in a different thread for the sake of courtesy.
(My 80m doublet is choked at the shack entry point.)
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