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Author Topic: End Fed Wire and Counterpoise  (Read 800 times)

N0AKX

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End Fed Wire and Counterpoise
« on: June 10, 2021, 03:42:16 PM »

Hi,

I am still confused on End Fed needing counterpoise. I have the Par End Fedz and the manual says counterpoise is NOT required. Which is true?

Also previous post says I may need RF Choke. How can I add choking on LMR-400?
« Last Edit: June 10, 2021, 03:45:33 PM by N0AKX »
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N6MST

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Re: End Fed Wire and Counterpoise
« Reply #1 on: June 10, 2021, 03:53:14 PM »

Hi,

I am still confused on End Fed needing counterpoise. I have the Par End Fedz and the manual says counterpoise is NOT required. Which is true?

Also previous post says I may need RF Choke. How can I add choking on LMR-400?

Many people use them without counterpoises. Many people use them with counterpoises. Each installation is different. Experimentation is your friend.

To add a choke to your coax, buy or build a choke and put it inline.
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W9IQ

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Re: End Fed Wire and Counterpoise
« Reply #2 on: June 10, 2021, 03:58:17 PM »

An end fed antenna requires a means of returning current to the transmitter. Most end fed designs rely on the exterior shield of the coax acting as that return path. So an end fed antenna is really an extreme off center fed antenna with the feed point between the wire and the braid. And the braid is the 'counterpoise'.

A common mode choke or two helps keep the shield common mode current out of the shack and out of the receiver as RFI. The design and location of the common mode choke(s) depends on the bands involved. In any case, an ugly balun (coax simply coiled up) is not recommended.

What are your bands of operation?

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

AA4PB

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Re: End Fed Wire and Counterpoise
« Reply #3 on: June 10, 2021, 04:10:21 PM »

If you are using the coax shield as the counterpoise then any choke needs to be near the transmitter end, perhaps just before entering the house. If you place a choke near the feed point then you will no longer have a counterpoise.
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Bob  AA4PB
Garrisonville, VA

N6YWU

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Re: End Fed Wire and Counterpoise
« Reply #4 on: June 10, 2021, 04:27:23 PM »

An end-fed can be thought of as an extremely off-center OCF wire antenna.  The impedance of an dipole or OCF depends on how far off-center the feedpoint is located.  From low (around 73 Ohms) near the center, to very high (several KOhms) near the end.  So you can better match your 9:1 or 49:1 transformer by choosing a short length of counterpose that moves your feed point away from the very end (of the counterpose) to a location (closer to the center of the end-fed) that matches the impedance of your transformer output.  That will reduce SWR losses, and allow you to put a choke on the feedline near the feedpoint to reduce common-mode currents on the feedline.  Two, three, or more chokes, spaced lambda/10 apart, will work even better,

You don't need the shield of the feedline to act as the current return path or the "other half of the antenna" (even though it will try to do that, as will any metallic objects in the near field), as you are feeding the antenna at a high voltage node, not a high current node, so there is very little current to return at that node.  The problem is the relatively high RF voltage around that node; so be careful about it's location, as it can be a potential a shock hazard.
« Last Edit: June 10, 2021, 04:32:25 PM by N6YWU »
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WA7ARK

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Re: End Fed Wire and Counterpoise
« Reply #5 on: July 18, 2021, 06:36:14 AM »

An end fed antenna requires a means of returning current to the transmitter.
Yes, and it uses the equal, but opposite-phase, self-cancelling, non-radiating currents that flow inside the feeding coax cable (coax Differential Mode currents) to do that.
Quote
Most end fed designs rely on the exterior shield of the coax acting as that return path.
No, no, no... With respect to creating a resonant, half-wavelength long "dipole" (or 1, 1.5, 2.0wl if the antenna is used on harmonics), only the first 0.07wl of the outside of the coax shield (nearest the transformer) must carry a small current (coax Common Mode current). If the feeding coax is at least 0.07wl long (~10ft for a 40m EFHW, longer if the antenna is designed to operate on 80m), that is all the "CM current on the coax shield counterpoise length" that the antenna needs to be resonant. That length is not critical; it can be longer, but it cannot be shorter.

If the feeding coax is longer than 0.07wl, put a ferrite coax CM choke (coax wound through a FT240-31 core) 10ft down the coax from the transformer, and the EFHW antenna will have a low
swr on all of its design bands. As far as the "antenna" is concerned, it now has everything it needs to operate.

Placing that CM choke at that 0.07wl spot down the coax eliminates most of the CM current that might otherwise flow down the coax shield all the way into the chassis of the tuner/rig/power-supply/AC-house-wiring. I use the word "most" because, even though the CM choke forces the CM current to zero right where it is placed, there can be a secondary CM current standing wave between the choke and the rig.

If that happens to a degree sufficient to be troublesome, it gets there because that part of the coax shield is a conductor that is physically in the near-radiated field of the dipole, and the current gets into the coax shield by magnetic induction. This is not unique to EFHW antennas.
« Last Edit: July 18, 2021, 06:39:04 AM by WA7ARK »
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Mike, WA7ARK

KM1H

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Re: End Fed Wire and Counterpoise
« Reply #6 on: July 19, 2021, 10:07:56 AM »

How many CM chokes are needed on very long feedlines, 100, 200, 300' or more of at least LMR 400 quality.

No OWL considered or wanted.

Carl
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WA7ARK

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Re: End Fed Wire and Counterpoise
« Reply #7 on: July 19, 2021, 11:03:06 AM »

How many CM chokes are needed on very long feedlines, 100, 200, 300' or more of at least LMR 400 quality...
In general, or specific to EFHW?

With EFHWs, lots of folks use no CM chokes and get by, on at least a few bands... That is because due to dumb luck, they just happen to use a coax length that avoids resonance at the frequencies that they operate. If they choose the wrong coax length and get RF-in-the-shack, then I would put choke #1 a few feet from the transformer, and try again. If they are still having problems, then placing a second choke close to the transmitter is step two.

Step 2 is less likely to be needed if the coax shield is earth-grounded (for lightning protection) before it enters the shack wall...

My philosophy on coax-fed dipoles and Yagis is to prophylactically put a CM choke at the feedpoint. On EFHW, put one at about 0.07wl (based on the lowest frequency that the EF works on) down the coax from the transformer.
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Mike, WA7ARK

W9IQ

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Re: End Fed Wire and Counterpoise
« Reply #8 on: July 19, 2021, 12:00:24 PM »

Quote
Most end fed designs rely on the exterior shield of the coax acting as that return path.
No, no, no... With respect to creating a resonant, half-wavelength long "dipole" (or 1, 1.5, 2.0wl if the antenna is used on harmonics), only the first 0.07wl of the outside of the coax shield (nearest the transformer) must carry a small current (coax Common Mode current). If the feeding coax is at least 0.07wl long (~10ft for a 40m EFHW, longer if the antenna is designed to operate on 80m), that is all the "CM current on the coax shield counterpoise length" that the antenna needs to be resonant. That length is not critical; it can be longer, but it cannot be shorter.

Yes, Mike I agree - that was sloppy wording on my part. Thanks for pointing that out. I should have said that most end fed designs rely on the exterior shield of the coax to function as the other part of the antenna.

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

KM1H

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Re: End Fed Wire and Counterpoise
« Reply #9 on: July 19, 2021, 03:45:01 PM »

Quote
In general, or specific to EFHW?

In general, Ive used a CM choke directly at the feed for various dipoles, inverted V's, a T2FD, and full 1/4 wave verticals with elevated radials from 160-10M since the mid 80's or so but not always at the same time.

One way to test a feedline is to terminate the far end with a commercial quality ~ 1W 50 or 75 Ohm load, screw on type N, etc with no chance of pickup. Then tune from say the BCB to 10M during excellent band conditions for any pickup (ingress). This is a first test before anything else is connected.

That is how I evaluate ~500' +  Beverage feedlines which includes several ground rods, a remote coax switch and the final run to the house is 1/2" CATV hardline. The runs from each Beverage to switches is RG6 quad shield and flooded. There are no remote preamps.

Never tried a EFHW but have an urge to try a very long inverted V type wire end fed and then to the top of the 180 or 100' tower and back down. I dont want any tuners involved, especially remote ones and this is no place to play with SS amps.

Carl
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AH7I

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Re: End Fed Wire and Counterpoise
« Reply #10 on: July 19, 2021, 06:24:02 PM »

Hi,

I am still confused on End Fed needing counterpoise. I have the Par End Fedz and the manual says counterpoise is NOT required. Which is true?

Also previous post says I may need RF Choke. How can I add choking on LMR-400?

You want the 1/2 wave wire resonant. The distance you feed it from the end leaves what some folks call a 'counterpoise'.

Feed line choke?  If you want/need, put it at the first high current point on the feed line, as you move away from the antenna. That's at the feed point on a dipole.  For your multiband voltage fed "EFHW" antenna, you may want more than one choke.

73, -Bob ah7i
« Last Edit: July 19, 2021, 06:27:25 PM by AH7I »
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