Where to put the Balun on an End-Fed Halfwave (#637)

Steven, N5SCG, is trying to set up his MFJ antenna but would like to know Dave's thoughts on how he should install it with his situation. Watch to learn more.


Edited by Callum Jakeman


Theme music is "Sour Tennessee Red," by John Deley and the 41 Players, courtesy YouTube Music Library.



Twitter: @dcasler

[hankhamner]

Hi Dave! I really emjoy your videos.

I have two EFHW antennas up 30 feet horizontal between two tall and rapidly growing Cottonwood trees. One is an 80-10 and the other an 40-10. Both have been working great continuously for 3 years and have withstood many days of high Oklahoma winds up to 60 mph mixed with the occasionally freezing rain and snow. The biggest problem I have with antenna wires or paracord in trees is that, unless you exercise the wire or cords back and forth every month or so when the trees are growing, the tree will grow over whatever antenna part is touching it making the antenna very hard to take down for any maintenance.

Needless to say I have learned a lot about EFHW antennas. I typically run about 700 watts on all bands except 10 meters and then about 300 watts max.

A main point to remember about these antennas is that the coax from the transformer box to the transceiver is part of the antenna system and carries a bunch of common mode current on the outer part of the shield. Both my transformers are up 30 feet and the RG213 heavy coax hangs vertically down and then across the ground about 25 feet to a three core choke Balun with a minimum of 7.5k resistive impedance over most of the lower bands and that virtually stops the common mode current before it enters my shack through an MFJ window mount. By the way, MFJ makes a fine meter that measures current on the coax shield. I have earned several awards using these antennas and many 5x9 voice contacts.

The choke Balun could be put right next to the transformer but then you would need a "counterpoise" to replace the shield part of the antenna. The counterpoise can be hanging from the transformer or laying on the ground. Although I have not tried it, most of the EFHW literature says the counterpoise attached ti the ground sde of the transformer should be about 5% of the lowest wavelength. For example 5% X 80 meters is 4 meters. It does not hurt or help to make it longer or to use more than one counterpoise.

The choke at the transformer will also slightly reduce the noise picked up by the coax shield. Final comment. If running more than 100 watts, wire or buy a 49:1 or 64:1 transformers that have 3-4 toroids of type 43 or 52. If you want to test your transformer make sure you test your resister. The wrong type resistor may show 2.45k ohms at 3 MHz and 400 ohms at 28 MHz.

[WHJBill]

It's an unun. You want to have at least 11 or 12 feet of coax from the transformer before you go to a surge arrestor and/or a 1:1 balun. That 11 or 12 feet will act as a counterpoise. As far as using that 49:1, or any non-weather proof system, I'd only use it for field work. Get a good weather proof 49:1 ... from MyAntennas, etc.

[goodnewsfromgod-bybob]

Dave, I’m beginning-not yet licensed- and just found and love your channel. I went back 6 years and started with your 1st. Up to #8 now. Thanks

[Ben--David]

Thanks Dave!
Yes Corpus Christi area we get high blowing rain. So I will have to drop the mast when a hurricane comes through. I am using a cable insulator for the wall penetration.

[Porco_Utah]

I have Inverted L, half wave length for 7MHz, I took PC board out of MFJ-993B metal box, and put in into water tight plastic case which are designed to hold AC power strip in garden. so I can place MFJ 993B directly at feed point. MFJ-993B was able to tune high impedance of halfwave without having any transformer. it actually tune 1.8 MHz all the way to 50 MHz. AG6JU

[ozone]

I would be worried about a lightning strike with the wire through the wall.

[kentkaee]

DO NOT put the unun inside the house. Run coax to a grounded lightning arrestor and then into the house. The coax is part of the antenna system and will radiate so I use a common mode choke before the coax enters the house. Rain will not hurt the unun, just mount the box so the vents face down so it drains

[arconeagain]

Dude, this is all over the shop. Like people have said, a decent coaxial ferrite choke is recommended. End fed antennas over a quarter wavelength in length suffer from significant CMC.

[the_mad_swimbaiter]

Very timely just /AG and looking to build an HF tenner

[justincase]

End fed antennas need grounding/counterpoise & common mode current chokes too …

[AKPJ]

Dave you said you wanted to have the unun close to the radio.. I have mine about 50ft from the house on a tree with about 80ft of rg8 (not ideal length I know). It's in an inverted V configuration with center roughly 35ft high.. The efhw had a lot of noise when I had it close to the house before.. I make lots of contacts all over the world with 0 noise floor after getting it out away from the house. but I'm always wondering if I can improve it for breaking pileups better. So I'm curious about your findings to keeping it closer to the rig?

[richardchandler]

So if the highest radiating point is at the balum end why would placing it against the house be a good idea? I think I remember the highest current/radiation is at the feed point and the highest voltage at the end of the antenna. Thoughts? And as always much thanks.

[coroamanicolai]

What about leaving the transformer in mid air ouside? Will that couse any problems while raining? Should it be covered so that no water gets inside?

[wjess]

My Question, Mr Casler, is... what difference does it make if the balun is at the top or bottom of a sloper? Have you looked at this? I bought an EFHW unun and would like to get it set up as I have no 40 meters. Which way to go, George? Which way to go? I know you won't answer this.. but, there is probably another expert out there.

[plato]

Don't forget a common mode choke.

[eknaap]

Feeding the transformer with coax (unbalanced) and making an EFHW (unbalanced) work, one would use a UnUn. Greetings from the Netherlands, PD0PSX

[DonzLockz]

I'm guessing the balun should be grounded too?🤔

[viewnys]

Where is the lightening arrestor in this antenna arrangement???

[tomcook]

I hit dislike buy mistake, I hit like right away though