Best way to construct half wave dipole w/coax feed?

[Todd, KA1KAQ]

Please excuse my ignorance here, but feedline/antenna impedance in general has never been my strong suit. Too many variables to confuse my mathematically-challenged mind.

I'm thinking band-specific antenna here, so the benefits of open wire for multiband use aren't really an issue. In the past I've just fed them directly with coax, no balun involved. An OT friend insists this is fine since the impedance should be at or around 70 ohms if built and hung correctly. He even suggested running RG-11 instead of RG-8, 213, etc. without a tuner since the Pi-network should tune the transmitter FB.

However, since I'm now right smack in the middle of a town/village here that is no doubt filled with cheap asian consumer junk (poorly filtered or shielded phones, TV, audio, etc), I want to be sure that I'm not broadcasting to everyone around me when I get on the air. Not that it won't happen anyway, simply to be sure I'm doing all I can to prevent my station from being at fault.

A number of years ago I did pick up some of those large ferrite beads that slide right over RG-8 size coax, I think maybe a half dozen of them. Will these do the trick or am I still better off adding a balun, even multiple coils at the feedpoint? My main concern is using moderate power levels and the antenna/feedline's ability to handle it without causing local RFI.

When I was last on the air regularly it wasn't so much of a big deal. There wasn't as much junk out there and I could just chop off a random run of coax, hook it directly to the dipole via center insulator, and use a tuner inside if needed. Other than working one phone down in town with a line-resonance issue, there were no complaints. I was out away from civilization moreso than now though, and I want to be able to run a level of power that allows me to be heard and not be a pissweak liability when the bands are noisey.

We'll be moving next year, then I'll be able to play with some of the real antenna ideas you guys mention on here. Until then I just want to get back on the air as simply and 'clean' as possible.

[Jim, W5JO]

Balanced Line vs. Coax, the great debate.  One frequency dipole, meaning cut for, say 3.885?  If so and you don't change frequency, then balanced line is balanced and the opposite currents  cancel..  If you change frequency, the antenna will show a bit of reactance which will show up on one leg of the balanced feedline, meaning it is unbalanced.  Same thing for coax in theory.

Having had both and living in an area where there are ice storms, I went with the coax and bead setup.  Do you know the mix of the beads you have?  As I recall mix 43 or 75 is what they should be (someone correct me if I am wrong).  Considering the size of the beads and mix of the ones you have 5 or 6 of them is sufficient for 80 meters.  It will require 10 of them for 160.

In my overall scheme of things, coax and beads was the best overall choice and least costly.  Depending on your transmitter's design,the pi net should tune from 50 to about 150 ohms for the frequency you are choosing, maybe more than 150, some would even go to 300 ohms.  So you should be good to go if your antenna is a flat top at least 1/4 wave or more above ground. Should it be lower, all bets are off.  If it is a flat top up 1.4 wave it should be close to 70-75 ohms and will work with either 50 or 75 ohm coax.  With 50 ohm coax you will see about 1.3:1 SWR, which is not a killer.

So a flat top up 1/4 wave, with moderate power and either a good balun or beads on it, will possibly overload some of the crappy stuff your neighbors have.  Can't help it.  Unless you have a setero nut close, probably the most trouble with be phones and answering machines.

Curing interferency troubles is a topic for a book and you didn't ask  me to write that and please don't.  But I will guess you will experience some RFI troubles close by with 100 watts out.  The higher in power you go, the farther the trouble will be.  And you can have the cleanest signal in the world to find you will interfer with someone in a close neighborhood.  I just hope they are tolerant and will let you work the problem out.

[W2VW]

If you already have a  balanced line feed that is run properly and a good antenna coupler then there is no advantage to coax feed from an interference standpoint.
I switched to balanced feed here to get rid of interference issues. It worked.

[Steve - WB3HUZ]

I ran an open-wire fed dipole at about 50-60 feet high for many years in a densely packed neighborhood (lot sizes around 0.25) acres and above ground power and telephone lines. I ran a maximum of 350 watts output on AM. The only RFI problems I had were with telephones. Filters fixed the problem. Coax may lessen the problem, but I would guess, unless the open-wire/ladder line is severely unbalanced (and I don't know why it would be if installed properly and used with a good tuner), it shouldn't contribute much more to the RF field than coax. YMMV.

But, if you are only planning to use the dipole on one band, coax is a little easier to deal with. At my last QTH, I used coax-fed dipoles (one for 160 and another with two dipoles off one feedline for 75 and 40 meters). I had zero RFI problems. The houses were much further apart at this location, 50-100 yards and the power and telephone lines were under ground.

[KB2WIG]

ID in SSB   .... it will cut down lots of complaints.    klc

[K1JJ]

Hi Todd,

Use a 4" PVC or ABS pipe and coil coax turns on it as an RF choke.  4 turns for 10M  .... 16 turns for 75M should do it.

This gives a 1:1 balun.


T

[VA3ES - Piss-Weak Ed]

Or, you can use a <A href="http://www.radioworks.com">Radio Works[/i]</a> dipole current balun in place of the 84' of RG-213.

A  horizontal dipole is approx. 72 ohms Z, an inverted V is approx 52 ohms Z.  The dipole's 1.3:1 SWR shouldn't matter too much.

[Glenn NY4NC]

Exactly what I did with all my single band coax fed dipoles at this QTH Todd. I slid beads over about a two and a half foot section of coax right at the feed point. Coiled up that section of coax so it would fit inside a large diameter piece of PVC pipe and sealed it up. This also acted as a center insulator with two eye hooks on either side of the pipe at the top. The only probably with this arrangement is it becomes too heavy for a dipole suspended end to end. Has to be an inverted vee or dipole supported in the center.

As Tom mentioned, a coil of coax works, The beads method was a better (easier/cheaper) option for me. (got the beads for free!) 

A number of years ago I did pick up some of those large ferrite beads that slide right over RG-8 size coax, I think maybe a half dozen of them. Will these do the trick or am I still better off adding a balun, even multiple coils at the feedpoint? My main concern is using moderate power levels and the antenna/feedline's ability to handle it without causing local RFI.

[k4kyv]

ID in SSB   .... it will cut down lots of complaints.    klc

Hey! that sounds like an auxiliary slopbucket rig might serve some useful purpose after all!

[Bacon, WA3WDR]

I recently had occasion to experiment with slightly-off-center coax feed to a half-wave dipole.

I have long believed there would be a zero-point for coax feed, and indeed there is.  On 145 MHz, I found if I offset the feedpoint about 6%, and connected the shield to the longer section, and the center conductor to the shorter section, I got a pretty good null on the shield.  On 75 meters, that would be a little bit more than 7 feet off center.  I'll get better figures as I experiment more.

It's interesting that this is about the same length-offset that is used for a ground plane's ground radials.  Maybe achieving a zero-point on a coax shield is the reason that this is done.

[K1JJ]

Or, you can use a <A href="http://www.radioworks.com">Radio Works[/i]</a> dipole current balun in place of the 84' of RG-213.

Hi Ed,

Does it use a toroidal transformer balun, ie, wire wound on the balun in a true transformation, or does it work as a choke?

Transformation toroid baluns are used all the time. The StackMatch is a good example where toroids are used up to 6M with good results for matching unbalanced to balanced systems.  Chokes aree a different story, but still work "OK", but not perfectly.

Read: 
http://amfone.net/index.php?pid=12

I would rate the effectiveness of reducing feedline radiation in order - starting with the best method. This is only MY opinion:

1) BEST: True 4:1 COAXIAL 1/2 wave transformation balun cut for the operating freq - measured with an ant analyzer. [bulletproof to everything - EMP, power saturation - limited by melting at 10KW power level...    True transformer- no reason for chokes if used. Disadvantage: 4:1 ratio only - Good for one band only.

2) REAL torroid transformer balun. Real transformer with wire windings - and allows for a 1:1, 1.5:1, 2:1 step up, etc... flexible for matching. But, EMP susceptible.  Advantage: Broadband for 1.8-10M with some designs.

3) Choke methods: Use BOTH Ferrite Beads AND coaxial coil [hank of turns, not a transformer above]  A band-aid approach since the coax is still unbal and the dipole is bal... no transformation.  Broadband.

4) Lots of big ferrite beads (IF they are the proper permability for the freq and large enuff not to saturate)  They are susceptible to permanent EMP saturation, however. Braodband.

5) Coiled Coax turns alone at the feedpoint.  Broadband, but coiled hank tends to have inter-cap problems above 40M. Coil coax turns on a PVC form for a better coil above 40M.  160-40M OK to coil in a hank and tape.

6) W2AU choke ferrite type "baluns".  Smoke city on QRO AM.

7) Direct feed, no choke, with feedline coming away at perfect 90 degrees.

They all "work" - just a matter how type A you wanna be. And, individual physical layouts will change results.  Houses, other ants, departure from straight and flat textbook dipole form, etc.

Will there be much difference between #1 and #7 for a dipole on 75M? Probably not. The #7 pattern  may be skewed slightly, but the on-the-air results will be good either way. Where it really begins to matter is when you have a directional antenna, like a Yagi that wants to see a nice pattern - nice front to back and clean frontal lobe. But, where it really is critical to have near zero feedline radiation is when stacking antennas/Yagis. These inbalances can wreak havoc on the phase relationship between antennas and can often make a good system perform poorly.

But my philosophy has been, if it takes only a small EXTRA effort to make something near "perfect", why not? A db here, a db there - after a while they add up and give an edge over the station who let the smaller details pass by.

T

[WA3VJB]

5) Coiled Coax turns alone at the feedpoint.  Broadband, but coiled hank tends to have inter-cap problems above 40M. Coil coax turns on a PVC form for a better coil above 40M.  160-40M OK to coil in a hank and tape

This one has worked well for me since Tom recommended it about five years ago.

[Jim, W5JO]

5) Coiled Coax turns alone at the feedpoint.  Broadband, but coiled hank tends to have inter-cap problems above 40M. Coil coax turns on a PVC form for a better coil above 40M.  160-40M OK to coil in a hank and tape

This one has worked well for me since Tom recommended it about five years ago.

The beads weigh less and you can use a standard dog bone insulator.  Tie the support rope to it and haul it skyward.

[Glenn NY4NC]

Wow!.... is that a 160ft coil of RG-213 up at your feedpoint for 160 meters?

1) BEST: True COAXIAL 1/2 wave transformation balun cut for the operating freq - measured with an ant analyzer. [bulletproof to everything - EMP, power saturation - limited by melting at 10KW power level...    True transformer- no reason for chokes if used. Disadvantage: Good for one band only.

[Todd, KA1KAQ]

Thanks for all of the tips, guys. I've been AM radio-inactive for the most part since '94 and as a result, I've forgotten the little bit I knew about antenna-magic-stuff. To be honest, it was always a head-scratcher for me. No problem with the tri-bander, or pre-built gamma match type stuff, but beyond that....?

To clarify a bit: the ferrite beads were given to me by another ham who collected them doing satellite work (pre-'dish'). They are roughly 1.25" long, maybe .25" thick and will slide over RG-8 sized coax. The 'mix' or composition is unknown, I assumed they must work since the guy who gave them to me used them also (how's that for scientific basis?).

There are a few other issues that play into this, which is why I want to build the antenna for minimum interference potential. First, my property is a lot like sitting a shoebox on edge, with the house on top but surrounded by trees on one side and behind, houses and businesses all around, main street and power lines/street lights out front. To make matters worse, parts of the back roof are metal, and I don't have a really good place to hang a dipole in the clear or flat. The antenna will pass within 15-30 feet of the roof. So I'm sure that as well as finding the junk electronics in the neighborhood, there is potential for all kinds of garbage from my end if not careful.

On the good side, I can build a dipole specifically for 75 meters and perhaps another later for 40. I have a great, heavy duty half-wave 160 ready to go but no place to hang it. So for now I want to get something up that will handle a few hundred watts while causing minimal problems. Basically, I don't want my property vandalized when I'm not home. 

I would lean towards open wire line if I could run it properly, but the twists and turns and eventual contact with other things preclude that. I have a Johnson KW matchbox that will work for that once I move. In the meantime, I have one of the Millen tuners made for coax, so I can use that.

Fortunately, Hopkinton is just around the corner so I can pester some of you in person.    I can draw crude pictures (of my location, that is) along with antenna orientation and the path the feedline has to follow. In the end though, I'll use the best method suggested that I can manage and go for it. I'm not going to be off the air for another winter. Listening is fine, but it gets old.

Hey Glenn, since you haven't moved down to NC yet, will you be making the trip to NH for Hosstraders?

[K1JJ]

Todd:

For the dipole, just put it up as flat and straight as possible. That's the goal around the obstacles you have. Make the feedline come away as close to a right angle as possible... try for at least 35', better to get a 65' of straight 90 degree run if possible.  Remember that no matter what we have up, everyone wishes they could have a better installation, but we work with what we have at the moment.

As for the ferrite beads... they are used from power supply freqs way up into VHF+. They all have different permabilities. You use beads with high permability on VHF, but when used on HF, they saturate easily and are of no use. Vice versa, a low perm HF bead does not have enuff perm core to work up on VHF without using a ton of them.  So, try to determine what you have, or just buy them for HF use.  Otherwise, you may put them on thinking they are working, but have no way to determine it in practice without some A/B tests that take time.

Glenn:  Yes, 168' of RG-213 for 160M. That's why the coaxial balun is not used much above 40M except for extreme "must do" antennas like Yagis. Personally, I simply use a coiled hank for my 160M and 75M dipoles. But on the 40M Yagi you can bet I use the 40' coaxial balun. If I put up a 75M Yagi, I would also use the 85' balun..   Again, for a simple dipole that looks for general coverage, a set of beads or coiled hank is all ya need to obtain a great pattern.

BTW, on 6M the coaxial balun is only 6.5' of coax, so very cornvenient.

Paul:  Yes, I remember our conversation where you put up a 10M dipole [at 60' high?] using the PVC pipe form to wind the coaxial choke. From hearing your contacts via back scatter at times, it appeared it worked out pretty well for you.

73 all,
T

[VA3ES - Piss-Weak Ed]

Or, you can use a <A href="http://www.radioworks.com">Radio Works[/i]</a> dipole current balun in place of the 84' of RG-213.

Hi Ed,
Does it use a toroidal transformer balun, ie, wire wound on the balun in a true transformation, or does it work as a choke?

Tom, according to the Radio Works catalog, it's Guanella type balun, using "ferrite", so I assume it's toroidal.  They also claim it's a true "choke" balun rather than the more common voltage balun.  There's more info on their website, and catalog.
I use one of their Y1-5K  "choke"  baluns in place of the original BN86 voltage balun that came with my TH3, and indeed the pattern is more symetrical, and there's less coax radiation.

These guys specialize in current baluns for various applications.  I believe thay also sell a true "Maxwell" beaded current balun (using a length of beaded coax) as well.

[VA3ES - Piss-Weak Ed]

BTW - speaking of antennas, my tower is up, and the beams and verticals are installed thereon. All that's left to do is the dipole, which will hang off the top of the tower (at about 60') and run down to a light pole in the street at one end, and a tree on the other.  The ends will be at approximately 35-40' or so.

All that's left is to move the shack to the basement rec-room, hook up the antenns, and we'll be piss-weak  no more!!!

[W2VW]

BTW - speaking of antennas, my tower is up, and the beams and verticals are installed thereon. All that's left to do is the dipole, which will hang off the top of the tower (at about 60') and run down to a light pole in the street at one end, and a tree on the other.  The ends will be at approximately 35-40' or so.

All that's left is to move the shack to the basement rec-room, hook up the antenns, and we'll be piss-weak  no more!!!

'Bout time. Last time I heard you on was when you were takling to Joe Walsh on 29.050.

[VA3ES - Piss-Weak Ed]

BTW - speaking of antennas, my tower is up, and the beams and verticals are installed thereon. All that's left to do is the dipole, which will hang off the top of the tower (at about 60') and run down to a light pole in the street at one end, and a tree on the other.  The ends will be at approximately 35-40' or so.

All that's left is to move the shack to the basement rec-room, hook up the antenns, and we'll be piss-weak  no more!!!

'Bout time. Last time I heard you on was when you were takling to Joe Walsh on 29.050.

Hah!  I recall that!  Musta been about four or five years ago!!!  Was using the End-fed Zepp at the time.  Well, we're all geared uyp for ten now, in high-style.  Have the beam, and a nice modified CB 5/8ths now on ten.  I modified the 5/8ths by simply shortening it. Put the MFJ 259B on her and she's 1:1 at 28.800, with zero reactance!  That antenna will deliver about 3.8 dB gain, and we'll use it for general 10M work.

[Glenn NY4NC]

Hi Ed,

Be careful about much power you run into that 5/8 ant. I had a Hy-Gain "colinear II" 5/8 wave good buddy ant some years ago, burnt out the matching coil running 100w on 10meters. Opened it up and found the matching coil in the base consisted of a flat PC board etched coil. Very thin narrow copper trace. 

BTW - speaking of antennas, my tower is up, and the beams and verticals are installed thereon. All that's left to do is the dipole, which will hang off the top of the tower (at about 60') and run down to a light pole in the street at one end, and a tree on the other.  The ends will be at approximately 35-40' or so.

All that's left is to move the shack to the basement rec-room, hook up the antenns, and we'll be piss-weak  no more!!!

'Bout time. Last time I heard you on was when you were takling to Joe Walsh on 29.050.

Hah!  I recall that!  Musta been about four or five years ago!!!  Was using the End-fed Zepp at the time.  Well, we're all geared uyp for ten now, in high-style.  Have the beam, and a nice modified CB 5/8ths now on ten.  I modified the 5/8ths by simply shortening it. Put the MFJ 259B on her and she's 1:1 at 28.800, with zero reactance!  That antenna will deliver about 3.8 dB gain, and we'll use it for general 10M work.

[VA3ES - Piss-Weak Ed]

Hi Ed,

Be careful about much power you run into that 5/8 ant. I had a Hy-Gain "colinear II" 5/8 wave good buddy ant some years ago, burnt out the matching coil running 100w on 10meters. Opened it up and found the matching coil in the base consisted of a flat PC board etched coil. Very thin narrow copper trace. 

Not likely with this antenna - it uses a hair-pin match, consisting of an external matching section hair-pin match.  The antenna manual says it will easily handle 1500 W. There is no internal coil.  In any case, I don't intend to run much above 100 watts into it.  This antenna is the "Penetrator", model #500.