Here's my understanding of common-mode current on coax feedline with an EFHW: An UNUN feeding an EFHW is unbalanced by design. For an attached coax cable, the cable shield's inner surface and center conductor's outer surface carry differential currents. But the cable shield's outer surface can carry a common-mode current. The common-mode current may be induced by near-by electrical devices (motors, dimmer switches, etc.) induced via the unbalanced nature of the antenna feed, induced by the EM near-field of the fed antenna, induced by the EM near or far-field of a relatively close high-powered RF emitter, or all four sources (i.e., a multi-source or composite current). For the EFHW antenna, the "counterpoise" current is a common-mode current (i.e., it flows on the outer surface of the shield.) This common-mode current can be greatly attenuated by placing a "common-mode choke" (e.g., multi-turns of coax wrapped around a toroid) between the transmitter/receiver and feedline. Ken WA8FCI
My definition of counterpoise is that it is anything that functions as a RF ground. It's relationship with earth ground is determined by how it is mounted in relation to ground. You could call the ground side of a dipole a counterpoise. You could also call the hot side of a dipole a counterpoise because it works like a radiating ground the same as the other side. The term "RF gound" is confusing because it sounds like a dc ground but it isn't. For all purposes, an RF ground is just something where RF energy will flow. If it radiates, it does not matter. What matters is that RF current will flow into it and has an RF impedance.
Thank you for a very informative video linking several topics about antennas. Would you consider doing an additional video or QST "Ask Dave" answer discussing the relationships between SWR, antenna resonance, and antenna efficiency? Much appreciated!
Hi, I’ve been watching your channel for a couple of years, really informative, thank you! My question is specifically on EFHW antennas. I have a Vibroplex shortened 80/10 which uses a coil as an inductor trap and loading for the 80m section, but this only gives a narrow 75KHz usable bandwidth without a tuner. The antenna is a half wave on 40m from the coil to the feedpoint. I use the outer braid as a counterpoise and have a 1:1 choke about 55 feet from the feedpoint and a lighting arrestor a few feet further away at about 65 feet from the feed point. Does the counterpoise (coax braid) radiate as powerfully as the radiating wire? I’m thinking that on 40,15 and 10m the full half wave length of the radiator will allow the wave to form on those bands but for 80m outside the narrow bandwidth provided by the coil, some of the signal may be forming on the counterpoise? I may have this all wrong! Always learning in this hobby! Thank you, Jim 73 from the UK
Would love to see if gauge of wire matters for radials. I see alot of 18awg radials, but there is alot of 26awg. That is a huge difference. for portable use with couple dozen radials, thinner would be better and cheaper BUT is 26awg as effective as 18awg. If not, by how much?? negligible? like 15watts vs 20 watts or is it like 15watts to 45 watts?
I have an MFJ 931 artificial counterpoise attached to a junior g5rv. The meter shows full current when transmitting. My shack is on second floor (loft), so cannot use a ground due to distance to transceiver. I don't see any improvement in rf tx rx performance though. Many thanks for your excellent videos. You are a great teacher.
scary isn't it. you can feel RF bite through your sweaty palm with only 5 watts out. Imagine some Chinese hh's advertising 15 watts.. woohoo. Cooking time.
I have an EFHW with no counterpoise and a 50 foot coax. In a previous video (last year I believe) you were somewhat adamant about the fact that you needed to use a counterpoise with EFHW. In this video you said you don't need one because your 50 foot coax acts as one. I am curious as to why you changed your position? Am I remembering this wrong? I have watched almost all of your videos, so I apologize that I cannot recall which one it was. Thank you for what you do.
For an EFHW, the length of your counterpoise need only be 0.05 of a wavelength. Odds are, if you have pretty much any length of coax on an EFHW, you don't need a separate wire for a counterpoise, the shield on your feed coax will work. So just make sure you don't put a choke right next to the transformer. I've tried adding a dedicated counterpoise to my 80-10 EFHW, which has 70 feet of LMR-400 feeding it, and it made zero difference.
Well Dave, if the products of a common current traveling back down the feed line towards the transmitter, are made up of the same components as the original, ( AC ), then where as they don't go thru some metamorphosis, and not knowing their names, Differential, or Common mode why doesn't the transformer allow those like components to pass, equally, and free of interference ? How would a, " Dumb Choke ", know the difference ?
12 minutes of a boring topic time to unsubscribe. To each his own. Where's all the commentary. Antennas I know very well matter of fact. I use to install and tune them for my friends years ago. Then the pagers came out followed by the prepaid cellphone still to this day. People don't know the difference between prepaid and postpaid Duh. I remember full quarter wave on cars for AM. Town of mayberry if you want to have a look. Sheriffs car.
Here's my understanding of common-mode current on coax feedline with an EFHW: An UNUN feeding an EFHW is unbalanced by design. For an attached coax cable, the cable shield's inner surface and center conductor's outer surface carry differential currents. But the cable shield's outer surface can carry a common-mode current. The common-mode current may be induced by near-by electrical devices (motors, dimmer switches, etc.) induced via the unbalanced nature of the antenna feed, induced by the EM near-field of the fed antenna, induced by the EM near or far-field of a relatively close high-powered RF emitter, or all four sources (i.e., a multi-source or composite current). For the EFHW antenna, the "counterpoise" current is a common-mode current (i.e., it flows on the outer surface of the shield.) This common-mode current can be greatly attenuated by placing a "common-mode choke" (e.g., multi-turns of coax wrapped around a toroid) between the transmitter/receiver and feedline.
Ken WA8FCI
My definition of counterpoise is that it is anything that functions as a RF ground. It's relationship with earth ground is determined by how it is mounted in relation to ground. You could call the ground side of a dipole a counterpoise. You could also call the hot side of a dipole a counterpoise because it works like a radiating ground the same as the other side. The term "RF gound" is confusing because it sounds like a dc ground but it isn't. For all purposes, an RF ground is just something where RF energy will flow. If it radiates, it does not matter. What matters is that RF current will flow into it and has an RF impedance.
Thank you Dave, you are a great teacher.
Thank you for the video. What location of a vertical antenna is where most of the radiating takes place?
For quarter wave radials at full length, the formula is for a quarter wavelenghth (234/f (MHz). You quoted the numbers for a half wavelength... - AL7W
Good catch! Thank you.
Thank you for a very informative video linking several topics about antennas. Would you consider doing an additional video or QST "Ask Dave" answer discussing the relationships between SWR, antenna resonance, and antenna efficiency? Much appreciated!
does the counterpoise have anything to do with SWR, or does it just affect Rx?
It increases Rx and decreases SWR significantly, when done well.
Dave, thanks for all your videos.
Hi, I’ve been watching your channel for a couple of years, really informative, thank you!
My question is specifically on EFHW antennas.
I have a Vibroplex shortened 80/10 which uses a coil as an inductor trap and loading for the 80m section, but this only gives a narrow 75KHz usable bandwidth without a tuner.
The antenna is a half wave on 40m from the coil to the feedpoint.
I use the outer braid as a counterpoise and have a 1:1 choke about 55 feet from the feedpoint and a lighting arrestor a few feet further away at about 65 feet from the feed point.
Does the counterpoise (coax braid) radiate as powerfully as the radiating wire?
I’m thinking that on 40,15 and 10m the full half wave length of the radiator will allow the wave to form on those bands but for 80m outside the narrow bandwidth provided by the coil, some of the signal may be forming on the counterpoise?
I may have this all wrong! Always learning in this hobby!
Thank you,
Jim
73 from the UK
In general, the counterpoise doesn't count as the radiating part of the antenna, but each situation is different.
You're correct. Ken WA8FCI
Would love to see if gauge of wire matters for radials. I see alot of 18awg radials, but there is alot of 26awg. That is a huge difference. for portable use with couple dozen radials, thinner would be better and cheaper BUT is 26awg as effective as 18awg. If not, by how much?? negligible? like 15watts vs 20 watts or is it like 15watts to 45 watts?
Generally, thicker guage is better.
David what’s the importance of the capacitance, in a transmission antenna? Thanks.
I have an MFJ 931 artificial counterpoise attached to a junior g5rv. The meter shows full current when transmitting. My shack is on second floor (loft), so cannot use a ground due to distance to transceiver. I don't see any improvement in rf tx rx performance though. Many thanks for your excellent videos. You are a great teacher.
what would happen if u put 4 long wires on the endfed n s e w and a counterpoise in each direction??? would that work?
Is it true that with hand held radios, the operator is the counterpoise?
scary isn't it. you can feel RF bite through your sweaty palm with only 5 watts out. Imagine some Chinese hh's advertising 15 watts.. woohoo. Cooking time.
I have an EFHW with no counterpoise and a 50 foot coax. In a previous video (last year I believe) you were somewhat adamant about the fact that you needed to use a counterpoise with EFHW. In this video you said you don't need one because your 50 foot coax acts as one. I am curious as to why you changed your position? Am I remembering this wrong? I have watched almost all of your videos, so I apologize that I cannot recall which one it was. Thank you for what you do.
For an EFHW, the length of your counterpoise need only be 0.05 of a wavelength. Odds are, if you have pretty much any length of coax on an EFHW, you don't need a separate wire for a counterpoise, the shield on your feed coax will work. So just make sure you don't put a choke right next to the transformer. I've tried adding a dedicated counterpoise to my 80-10 EFHW, which has 70 feet of LMR-400 feeding it, and it made zero difference.
This is the first time I wished I could give a video TWO thumbs up (or even 3).
Well Dave, if the products of a common current traveling back down the feed line towards the transmitter, are made up of the same components as the original, ( AC ), then where as they don't go thru some metamorphosis, and not knowing their names, Differential, or Common mode why doesn't the transformer allow those like components to pass, equally, and free of interference ?
How would a,
" Dumb Choke ", know the difference ?
What is a ground counterpoise?
Haircut helps a lot!!!
I thought a counterpoise was usually a piece of coax tuned for capacitance
You make me muse about things Ham. I like that Dave.
A counterpoise is an antenna for your antenna .is what a really old ham told me once. I never argued with him I said ok sounds good to me.
I was "with you" until you used the phrase "something for the antenna to work against.". Say that in a more descriptive way, please
The title of this vid is a great pun!
So knowledgeable…..
Still proud that you voted for Biden Dave? It wouldn't surprise me if you were lmao.
How do you know Dave voted for Biden?
12 minutes of a boring topic time to unsubscribe. To each his own.
Where's all the commentary.
Antennas I know very well matter of fact. I use to install and tune them for my friends years ago. Then the pagers came out followed by the prepaid cellphone still to this day.
People don't know the difference between prepaid and postpaid Duh.
I remember full quarter wave on cars for AM.
Town of mayberry if you want to have a look. Sheriffs car.
Sooner you unsubscribe the better.
10 Subscribers...Easy to take potshots...'Nuff Said'. Hope you get better. God Bless.