Help with 1:1 balun for dipole

[KJ6MJS]

Hi. I've done a lot of research about how to wire a balun, but I'm left scratching my head and need some help.

I am building a dipole antenna for 3 - 30 Mhz. About 20 - 30 ft of RG58 50 Ohm coax will terminate at the balun, where it feeds two elements of 14 AWG THHN copper wire (coated) to form the dipole.

My question is (a) what type of toroid is required (eg, size, etc) and (b) how to wire around the toroid.

(a) Does it matter how big the toroid is? I would like this to fit into 1 1/2" PVC tubing. Also, is any particular rating of toroid required?

(b) Is it OK merely to take the two strands of wire from the SO239, wrap it around the toroid six times, and then feed the 14 AWG wire elements (one wire from the core, one wire from the shield)? I've seen a number of designs that have unusual wiring patterns. 27/11 - Baluns

Thanks.

 

[prcguy]

Are you making a 1/2 wave dipole at specific frequencies across 3-30MHz or making it one size to use across that entire range? For transmit or receive only?
prcguy

 

[K9WG]

Maybe this can help.

http://www.nonstopsystems.com/radio/balun-sevick.pdf

 

[KJ6MJS]

I will be using separate wires for each specific frequency. Both receive and transmit. Operating at between 20 - 100 watts for transmit.

I forgot to add that I intend to use 12 gauge enameled (magnet) wire for the balun itself.

 

[KJ6MJS]

Maybe this can help.

http://www.nonstopsystems.com/radio/balun-sevick.pdf

Thanks for the reference. That's one of the documents I read in my research before posting.

 

[rfradioconsult]

From your original post can we assume that your intent is to use this antenna for receiving only? If that is the case spend your money and take the wife out to eat, a balun will not help. If your intent is to transmit then you need to re-think your design and feedline method.

 

[KJ6MJS]

If your intent is to transmit then you need to re-think your design and feedline method.

Please elaborate. With the exception of the wiring for the balun (which I'm trying to figure out), this design is suggested by Sevick and used by each of the armed services.

See, USMC Field Antenna Handbook, http://www.armymars.net/ArmyMARS/Antennas/Resources/usmc-antenna-hb.pdf, Section 3-5, 3-7, and 3-10.

 

[rfradioconsult]

How many bands do you intend to incorporate into one antenna? 2-3 bands are doable but with more bands the interaction make it difficult to prune each band for minimum reflected power. Do you intend to use a tuner or feed your radio directly? How much horizontal real estate do you have, how high; how do you intend to support it? If your intent is to just hang it from each end, the wire you specify becomes a problem, soft drawn wire will strech and change the lengh thus detuning the antenna.

A lot of folks will build a dipole as long as you have the space, feed it in the middle with twin lead or ladder line and use a balanced tuner or a balun at the exterior and run a short coax lead to the unbalanced port on the tuner.

 

[KJ6MJS]

How many bands do you intend to incorporate into one antenna? 2-3 bands are doable but with more bands the interaction make it difficult to prune each band for minimum reflected power. Do you intend to use a tuner or feed your radio directly? How much horizontal real estate do you have, how high; how do you intend to support it? If your intent is to just hang it from each end, the wire you specify becomes a problem, soft drawn wire will strech and change the lengh thus detuning the antenna.

A lot of folks will build a dipole as long as you have the space, feed it in the middle with twin lead or ladder line and use a balanced tuner or a balun at the exterior and run a short coax lead to the unbalanced port on the tuner.

Ah, gotcha. My responses below. I am building a light weight, portable dipole for emergency field use (post earthquake), so I have plenty of real estate.

"How many bands do you intend to incorporate into one antenna?"

Probably one at a time, but at most three. I don't mind changing the wire, but would prefer not to change the balun for each frequency. If a different balun is needed for certain ranges of Mhz between 3 - 30 Mhz, then I'll consider which bands are most important to me.

"Do you intend to use a tuner or feed your radio directly?"

Coax directly into the radio. Since it's a field operation, I want to keep this as minimalistic as possible.

"how high; how do you intend to support it?"

15' mast in the center. The end of the elements will be about 3 feet off of the ground. This is an inverted dipole.

"feed it in the middle with twin lead or ladder line"

Yes, I've been debating about using that. Still, the loss from coax for a short run will be nominal and with a balun would seem to be a sound design.

 

[rfradioconsult]

Well gee wiz, Why would an individual with obvious intimate knowledge of the fine act of building antennas need to ask a question on the board. Just find you an old Hygain tape measure adjustable dipole.

 

[prcguy]

For the purpose described you don't need a balun but if you must I would recommend figure 5 in the excellent article that K9WG posted.

For up to 100w I use an Amidon FT-114 core with a #43 mix and 10 turns of RG-316 coax wound as shown in figure 5. This fits inside a 1 1/4" or larger PVC pipe cap. I use the FT-240-43 core and RG-142 coax for higher power but its too large and heavy for a dipole center feed balun. 10 turns on this core is good for 80 through 10m and probably higher.

If you need an antenna for when the SHTF I have an article somewhere on a rapid deploy "maypole" NVIS antenna using two cross dipoles to cover 40, 60 and 80m without a tuner and direct 50ohm feed.

The dipole elements are the guy wires for the antenna and you can also feed it with 300ohm TV twinlead and a tuner for all band operation and it has fairly low angle radiation on 20m and above. This antenna uses the smaller FT-114-43 core at the feedpoint to decouple the coax and keep feedline radiation down in NVIS mode.
prcguy

 

[prcguy]

The old Hy-Gain tape dipoles are fetching ridiculous prices these days, especially the version with smaller Lufkin style winders and associated with Collins equipment. Prepare to fork out $150 to $200 for a used one.
prcguy

Well gee wiz, Why would an individual with obvious intimate knowledge of the fine act of building antennas need to ask a question on the board. Just find you an old Hygain tape measure adjustable dipole.

 

[rfradioconsult]

Every KWM-2A that went to the field most likely had one with it.

Thank goodness I don't have that worry with my Mobit 3T RDP kit, which includes the fiberglass shipping box.

 

[prcguy]

Thank goodness I don't have to use a bulky fiberglass shipping box with either of my PRC-138's!

BTW, my KWM-2A portable shipping suitcase is not much bigger than the Mobat 3T RDP, but the Mobat probably has slight better specs in some areas...
prcguy

Every KWM-2A that went to the field most likely had one with it.

Thank goodness I don't have that worry with my Mobit 3T RDP kit, which includes the fiberglass shipping box.

 

[KJ6MJS]

For the purpose described you don't need a balun but if you must I would recommend figure 5 in the excellent article that K9WG posted.

For up to 100w I use an Amidon FT-114 core with a #43 mix and 10 turns of RG-316 coax wound as shown in figure 5. This fits inside a 1 1/4" or larger PVC pipe cap. I use the FT-240-43 core and RG-142 coax for higher power but its too large and heavy for a dipole center feed balun. 10 turns on this core is good for 80 through 10m and probably higher.

If you need an antenna for when the SHTF I have an article somewhere on a rapid deploy "maypole" NVIS antenna using two cross dipoles to cover 40, 60 and 80m without a tuner and direct 50ohm feed.

The dipole elements are the guy wires for the antenna and you can also feed it with 300ohm TV twinlead and a tuner for all band operation and it has fairly low angle radiation on 20m and above. This antenna uses the smaller FT-114-43 core at the feedpoint to decouple the coax and keep feedline radiation down in NVIS mode.
prcguy

PRC, I am in your debt. Thanks.

Yes, this is a when the SHTF antenna. I live in San Francisco. If you get a chance and find the NVIS article, that would be great. I am building an NVIS using PVC 1.5" pipe as the mast. I figure all of the repeaters will be dead a few days after an earthquake, and this is a good project for me, since I am a brand new ham (less than a week).

 

[LtDoc]

Considering the use of this particular antenna system, I think I wouldn't include a balun at all.
- 'Doc

 

[prcguy]

Still looking for the article and I may not have completed one but in the mean time here is a mock brochure for the project antenna. If you PM me an email address I can send some pics of how the innards were assembled, etc.

This particular antenna is unique with an NMO mobile mount on top and ground radial connections so you can screw on your VHF/UHF mobile antenna and give it some elevation using the same mast. The addition of the NMO mount was a little complex mechanically and I have versions without it. The 19" ground radials for the VHF/UHF side and guy stakes are stowed inside the upper mast assembly.

The HF portion is basically a pair of dipoles for 40/80m that are right angles to each other and the 80m dipole has a small insulator and removable jumper at the 60m resonant point. The wire elements with approx 30ft of string on each end are the guy wires, so everything you need except the radio and coax is stowed on or in the antenna assembly. There is also an FT-114-43 core balun with 10 turns of parallel wire at the feedpoint.

This design doesn't really need a balun but I like feeding this one with TV twinlead and it should help with feedline radiation on bands where the antenna is not resonant. I have several lengths of TV twinlead with BNC connectors installed for this purpose.

I like using the 16ft telescoping Buddipole mast but that might be too pricey for some and a PVC pipe mast is ok depending on height. Ideally you want this antenna up at 30ft but use 20ft as a goal and at least 3/4" or larger PVC pipe would be needed as a mast.

I made a version without the NMO mount and it had a hole in the top cap around 3/8" so it would slide over the tapered top of the various 33ft telescoping fiberglass masts on the market. With the right size hole it will stop a few feet from the top of the mast where its strong enough to support the assembly. In this case you have to carefully glue the balun core centered in the top cap because the tip of the mast goes right through the balun core.

Anyway, hope this is what you are looking for.
prcguy

PRC, I am in your debt. Thanks.

Yes, this is a when the SHTF antenna. I live in San Francisco. If you get a chance and find the NVIS article, that would be great. I am building an NVIS using PVC 1.5" pipe as the mast. I figure all of the repeaters will be dead a few days after an earthquake, and this is a good project for me, since I am a brand new ham (less than a week).

 

[KJ6MJS]

The NMO on top is a great idea. I was looking into using a fender mount to side mount the NMO. I really like your design.

I will look into the buddipole. However, for my first NVIS antenna, I really wanted to build it from scratch in order to learn the ropes. FYI, you can get a self supporting mast by:

1. Take 3 PVC t-connectors (slip x slip x slip). Place one in the middle with the "t" sticking straight up. The other two connectors should be placed on their sides with their "t" facing away at 90 degrees. I then cement the "t" connectors together (and added two bolts inside for extra strength).

Then the legs for the stand can be cut to any desired length and are interchangable.

The attached pics were taken with a 16 foot mast and are sitting on concrete on a 1 - 2 percent grade.

Still looking for the article and I may not have completed one but in the mean time here is a mock brochure for the project antenna. If you PM me an email address I can send some pics of how the innards were assembled, etc.

This particular antenna is unique with an NMO mobile mount on top and ground radial connections so you can screw on your VHF/UHF mobile antenna and give it some elevation using the same mast. The addition of the NMO mount was a little complex mechanically and I have versions without it. The 19" ground radials for the VHF/UHF side and guy stakes are stowed inside the upper mast assembly.

The HF portion is basically a pair of dipoles for 40/80m that are right angles to each other and the 80m dipole has a small insulator and removable jumper at the 60m resonant point. The wire elements with approx 30ft of string on each end are the guy wires, so everything you need except the radio and coax is stowed on or in the antenna assembly. There is also an FT-114-43 core balun with 10 turns of parallel wire at the feedpoint.

This design doesn't really need a balun but I like feeding this one with TV twinlead and it should help with feedline radiation on bands where the antenna is not resonant. I have several lengths of TV twinlead with BNC connectors installed for this purpose.

I like using the 16ft telescoping Buddipole mast but that might be too pricey for some and a PVC pipe mast is ok depending on height. Ideally you want this antenna up at 30ft but use 20ft as a goal and at least 3/4" or larger PVC pipe would be needed as a mast.

I made a version without the NMO mount and it had a hole in the top cap around 3/8" so it would slide over the tapered top of the various 33ft telescoping fiberglass masts on the market. With the right size hole it will stop a few feet from the top of the mast where its strong enough to support the assembly. In this case you have to carefully glue the balun core centered in the top cap because the tip of the mast goes right through the balun core.

Anyway, hope this is what you are looking for.
prcguy

 

[rickak]

Hi. I've done a lot of research about how to wire a balun, but I'm left scratching my head and need some help.

I am building a dipole antenna for 3 - 30 Mhz. About 20 - 30 ft of RG58 50 Ohm coax will terminate at the balun, where it feeds two elements of 14 AWG THHN copper wire (coated) to form the dipole.

My question is (a) what type of toroid is required (eg, size, etc) and (b) how to wire around the toroid.

(a) Does it matter how big the toroid is? I would like this to fit into 1 1/2" PVC tubing. Also, is any particular rating of toroid required?

(b) Is it OK merely to take the two strands of wire from the SO239, wrap it around the toroid six times, and then feed the 14 AWG wire elements (one wire from the core, one wire from the shield)? I've seen a number of designs that have unusual wiring patterns. 27/11 - Baluns

Thanks.

Dont need a toroid, you can make the 1:1 current balun with coax:

BUILD AN AIR WOUND 1:1 CHOKE BALUN FOR HF - THE UGLY BALUN!

Rick

 

[KJ6MJS]

Dont need a toroid, you can make the 1:1 current balun with coax:

BUILD AN AIR WOUND 1:1 CHOKE BALUN FOR HF - THE UGLY BALUN!

Rick

Rick,

Yep, that's looking like the easiest solution for e-comm. Initially, I thought that may be too heavy to carry (due to the extra 20' of coax and the PVC), but it's looking like the most fail safe design.

Hopefully, it's as effective with RG58 as it is with RG8 (all the photos are using RG8).