Length of counterpoise on end fed half wave

[N8TI]

Hi guys. Is .05 wavelength lenght good enough for a counterpoise on an end fed half wave? That was the theory I read a couple years back. Do opinions differ?

Joe

[PA1ZP]

Hi Joe

hat would depend on the way you feed the half wave and its impedance transformer.
Our halfwave verticals aluminium do not need counterpoise as they have a autotransformer that results in a perfect  R 50 and J0  impedance.

73 Jos 

[W1JKA]

      From three years actual experience both fixed and portable use of efhw antennas with homebrew coupler: I have never needed a counterpoise wire.I have tried different recommended lengths in past and found no difference with or without them,however I only operate at QRP power levels,wheather higher power levels have an effect on the use of counterpoise wires or not, I'll  leave it to the antenna gurus to debate.Check out AA5TB website for a clear explaination of this issue then try it for yourself and see.     HNY  Jim              

[WX7G]

I assume you are referring to a horizontal Zepp fed with open line or ladder line. The length to make an effective "counterpoise" depends on the length of the open wire feeder. Make the feeder 1/4 wavelength and the counterpoise will do nothing. Make the feeder 1/2 wavelength and a short counterpoise can be effective.

A 0.05 wavelength single wire counterpoise presents and impedance of 1100 ohms. That's not much of a counterpoise. Make it 0.1 wavelength and you have 550 ohms.

[K5LXP]

If you add a "counterpoise" to an end fed half wave, it's no longer an end fed half wave.


Mark K5LXP
Albuquerque, NM

[W5DXP]

Make the feeder 1/4 wavelength and the counterpoise will do nothing.

Yes, according to EZNEC, here's the current distribution on a 1/2WL end-fed with 1/4WL of open-wire (Z0=~600 ohms) feedline - no counterpoise required. One of the feedline wires is just left floating at the antenna. This causes common mode current at the antenna but not at the feedpoint if a good choke or link coupling is used. Feedline radiation is maximum at the antenna feedpoint (not a bad thing).

http://www.w5dxp.com/ZeppCrnt.JPG

[AA4PB]

In spite of what some mfgs will tell you, an end-fed 1/2 wave ALWAYS has a counterpoise. If you don't specifically provide one then the coax shield will act as the counterpoise. If you connect the end-fed wire directly to the radio (as with an Elecraft with a built-in wide range tuner) then the case of the radio and anything connected to it will act as the counterpoise.

For a suspended counterpoise wire, the "ideal" length would be 1/4 wavelength. That would provide the lowest impedance on the common side of the feed point and send the greatest amount of power into the antenna and minimize any RF on the outside of the coax shield. Because the feed impedance on an end-fed 1/2 wave is high, you can get away with a lot less counterpoise. The higher your counterpose impedance, the more current will flow in the outside of the coax shield, so its a trade off. Many get away with just using the coax shield as the counterpoise (which some mfgs advertise as "no counterpoise").

For every amp of current that flows into an end-fed antenna there MUST be an equal amount of current flowing in the counterpoise. Some may tell you that there is no current flowing into an end-fed, but that cannot be. Remember that P = I * E so if I = 0 then there is no power, regardless of how much voltage is present.

Another efficient feed system is to use a 1/4 wavelength (or odd multiple) of balanced ladder line with one side connected to the end-fed 1/2 wave antenna and the other side unconnected at the antenna end. This is called a zepp.

[W8JI]

AA4PB is right.

ALL half wave end fed antennas need a counterpoise. Anyone saying they have one that does not, is kidding himself.

For every single milliampere of common mode current forced into the end of any wire of any length, there has to be an exactly equal current flowing from some other conductor as common mode current. There is no possible way around this.

If I have an end fed half wave thin wire, the common mode impedance of that wire might be a few thousand ohms. SQRT of P/R.  100 watts would require SQRT 100/5000 = .14 amperes flowing back.

That .14 amps can only be transformed up in value along coax shields or some other form of counterpoise, unless or until it is spread out and dissipated.

This also causes a voltage at the counterpoise.

This is why, if you have a half wave end fed, it is almost impossible to get rid of common mode currents on the feedline or feed system without radials, or a near 1/4 wave long counterpoise and some decoupling.

Anyone telling you it can be done some other way is blowing smoke.

http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm


73 Tom 
 

   

[WB6BYU]

Anyone telling you it can be done some other way is blowing smoke.

Or smoking something.

Our halfwave verticals aluminium do not need counterpoise as they have a autotransformer that results in a perfect  R 50 and J0  impedance.

An excellent public announcement that you really do not understand how your antennas work.




The purpose of the counterpoise isn't to reduce ground losses (though it may do so - just not
to the same extent as with shorter verticals.)  It isn't to improve the SWR (though it does
affect the SWR / load impedance to be matched.)  The purpose is to reduce the common mode
currents on the feedline.

Kirchoff's law says the current flowing into a node must equal the current flowing out of it.
This applies to the antenna feedpoint:  if you have 140mA flowing into the antenna, there
must be 140mA flowing somewhere else.  If you had a perfect choke that would prevent
this from flowing on the feedline or anywhere else, then you wouldn't be able to get any
power into the radiator - it would be like having a broken wire on one side of a power cord.


In most cases the coax braid acts as a counterpoise / radial.  Because of the high impedance,
almost any choke balun will NOT be effective to prevent this.  That doesn't mean that the
operator will necessarily notice any problems, especially at lower power levels.  The lower
the impedance of the counterpoise / radials, the more current flows in them instead of on
the coax shield.  How long the radials need to be to reduce the common mode current below
a particular value depends on the power level and the impedance looking back down the
coax shield from the feedpoint.

Remember that, while the current may be low compared to that flowing at the feedpoint
of a quarter wave vertical, it is still capable of radiating the same power as the antenna
itself if it flows into a high impedance resonant circuit - for example, if you just have a
half wave of coax connected to an isolated radio.  The ideal case for minimizing common
mode current is to provide a high impedance on the coax shield AND a much lower
impedance radial / counterpoise / ground connection where most of the current will flow.


So what is the ideal length?  Probably a quarter wavelength if the purpose is to minimize
common mode currents.

What is the minimum length you can get by with?  That depends on your power level,
coax length, shack grounding, etc.  In some cases the answer is "none" if the common mode
currents don't cause any problems.  Otherwise you'll have to experiment and see what it
is in your specific installation.

[W5DXP]

This is why, if you have a half wave end fed, it is almost impossible to get rid of common mode currents on the feedline or feed system without radials, or a near 1/4 wave long counterpoise and some decoupling. Anyone telling you it can be done some other way is blowing smoke.

True - however, the common mode current magnitude on the feedline of a well-designed Zepp is less than 10% of the antenna current at the center of the 1/2WL wire, i.e. a very large percentage of the radiation is being emitted from the center of the 1/2WL wire and not a lot from the feedline. Most of the feedline radiation that does happen is from the upper half of the feedline close to the antenna with a lessor percentage (maybe 0.2 dB of total power) from the lower half of the feedline close to the transmitter.

The way a classic Zepp works is by causing common mode current equal to antenna current AT THE FEEDPOINT where it doesn't have a very large effect and its magnitude is only a fraction (<10%) of the antenna current magnitude at the center of the 1/2WL wire. 1/4WL down the line is a common mode current standing wave NODE, i.e. the common mode current is nearly zero at the source end of a well designed Zepp. A proper common mode choke (or link coupling) between the 1/4WL parallel series section will discourage common mode currents on the coax to the shack. Here's what EZNEC has to say about it. Note that Ampere's law and Kirchhoff's law are perfectly satisfied with (ideally) zero common mode current at the Z0-matched parallel/coax junction.

http://www.w5dxp.com/ZeppCrnt.JPG

[WB6BYU]

I think there are two discussions here:  a half wave wire with a base matching network
of some sort, and a Zepp feed.

The true Zepp feed would use a quarter wave open wire line feeding  half wave wire,
often with  low impedance link coupling at the transmitter end.  The use of the quarter
wave transmission line makes things more predictable than a fixed tuner with a random
length coax to the rig.

[KF7DS]

Anyone telling you it can be done some other way is blowing smoke.

Or smoking something.

Our halfwave verticals aluminium do not need counterpoise as they have a autotransformer that results in a perfect  R 50 and J0  impedance.

An excellent public announcement that you really do not understand how your antennas work.




The purpose of the counterpoise isn't to reduce ground losses (though it may do so - just not
to the same extent as with shorter verticals.)  It isn't to improve the SWR (though it does
affect the SWR / load impedance to be matched.)  The purpose is to reduce the common mode
currents on the feedline.

Kirchoff's law says the current flowing into a node must equal the current flowing out of it.
This applies to the antenna feedpoint:  if you have 140mA flowing into the antenna, there
must be 140mA flowing somewhere else.  If you had a perfect choke that would prevent
this from flowing on the feedline or anywhere else, then you wouldn't be able to get any
power into the radiator - it would be like having a broken wire on one side of a power cord.


In most cases the coax braid acts as a counterpoise / radial.  Because of the high impedance,
almost any choke balun will NOT be effective to prevent this.  That doesn't mean that the
operator will necessarily notice any problems, especially at lower power levels.  The lower
the impedance of the counterpoise / radials, the more current flows in them instead of on
the coax shield.  How long the radials need to be to reduce the common mode current below
a particular value depends on the power level and the impedance looking back down the
coax shield from the feedpoint.

Remember that, while the current may be low compared to that flowing at the feedpoint
of a quarter wave vertical, it is still capable of radiating the same power as the antenna
itself if it flows into a high impedance resonant circuit - for example, if you just have a
half wave of coax connected to an isolated radio.  The ideal case for minimizing common
mode current is to provide a high impedance on the coax shield AND a much lower
impedance radial / counterpoise / ground connection where most of the current will flow.


So what is the ideal length?  Probably a quarter wavelength if the purpose is to minimize
common mode currents.

What is the minimum length you can get by with?  That depends on your power level,
coax length, shack grounding, etc.  In some cases the answer is "none" if the common mode
currents don't cause any problems.  Otherwise you'll have to experiment and see what it
is in your specific installation.

Great explanation.....i have been studying the common mode issue from my S9 43' (wire) vertical (which has a remote tuner at feedpoint and a choke there too) and this really sums up what i am seeing with this particular length of wire.

Don KF7QZB

[N8TI]

"Another efficient feed system is to use a 1/4 wavelength (or odd multiple) of balanced ladder line with one side connected to the end-fed 1/2 wave antenna and the other side unconnected at the antenna end. This is called a zepp."

So, if I have a 1/4 counterpoise or ladder line feedline  for 40 meters (abt 33 feet long), when I go to use it on 20 meters, I have a half-wave long counterpoise or feedline which is, of course, not an odd multiple of a 1/4 wave on 20 meters as it would be a half wave counterpoise or feedline.  It would then seem that such a zepp for 40 meters would not be advantageous for 20 meters.

If this is so, what would be a good feedline length to use the antenna for 40 and 20?

Or I am not getting this.

What I am trying to do is figure out what lenght my end fed half wave should be and the lenght of the counterpoise (or the ladder line) should be for a portable operation where I am sitting at a picnic table out in the park working from 5 to 30 watts with battery power.  Given that you really can't know how your antenna is working when you just set up outside for some portable work, at least until you have been there a while, I wanted to at least get the theory correct. I understand that the tuner will load up practically anything, so I don't want to rely on just getting a good SWR match.

I'm learning something from the discussion, (I think), so your work has not been in vain.
Thanks,

Joe

 

[WX7G]

N8TI for portable operation as you describe (low power) I would use the feedline and rig as the counterpoise. Yes there will be RF current on these items but so what.

[W5DXP]

If this is so, what would be a good feedline length to use the antenna for 40 and 20?

Note that using a 40m Zepp on 20m without adjusting the length of the feedline results in a one wavelength wire, end-fed with 1/2WL of feedline, a worst case scenario.

But the solution is still the same - start with an odd number of 1/4WL for the feedline for each band. If 33' is 1/4WL on 40m, then use 16.5' or 33'+16.5'=49.5' for 20m. Then, by fine-tuning the lengths and observing an antenna analyzer, one can tune for antenna system resonance on one's desired frequency on each band.

A couple of DPDT knife switches make switching that extra 16.5' section in and out a piece of cake - or it can be done automatically with a 4PDT relay.