Re: How to MATCH a 50 Ohm antenna/coax to 75 ohm Coax/hardline

[ssbothwell SWL]

Ref: http://amfone.net/Amforum/index.php?topic=5199.msg39428#msg39428


sorry to resurrect an old thread but this seems like the sort of technique i might need to use when i setup my station. i'm planning out how i want to setup my station once i get a chance to get my license.   i plan to use a half-wave dipole with a 5w transmitter for QRP CW on the 80m band.

as i understand it a halfwave dipole is ~75ohm impedance, coaxial cable comes in 75ohm or 50ohm impedance, and the linear amp i am looking at expects a 50ohm load. i assume i need some sort of 75ohm to 50ohm impedance matching between either a 75ohm coaxial line and the transmitter or between a 50ohm coaxial line and the halfwave dipole. is that correct?

if so, would the transformer K1JJ mentions in this thread be appropriate for my situation?

[Steve - K4HX]

You will probably not need any such matching for your dipole. Yes, a dipole has a feedpoint impedance of 75 Ohms in free-space. Your dipole will not be in free-space but over real ground. On 75 meters, if it is less than 50-60 feet high, it will probably have a feedpoint impedance closer to 50 Ohms than 75.

Even if it did turn out to be 75 Ohms, the mismatch is not worth worrying about.

[ssbothwell SWL]

thanks for the tips! thats one less thing to worry about. should i still use a 1:1 balun to match the balanced antenna to the unbalanced coaxial cable?

when you talk about 50-60feet high do you mean elevation above the closest surface or the earth? i plan to mount the antenna on the roof of my studio which is on the second floor of a two story strip mall. it will probably only be a couple feet off the surface of the roof.

[K1JJ]

I agree with Steve that the dipole will be a FB match for 50 ohm coax on 75M.  It's really not that critical because it is a low band (75M) and a single element.  Be sure to wind ~ 10-12 turns of coax onto a  3-4" diameter (PVC pipe) to act as a choke at the feedpoint.

I really posted the 75:50 ohm, 1/12 wave unun for use when matching multiple elements or Yagis - to give perfect power divisions and transfers.  If one Yagi hogs power due to a mismatch, this improper phasing may wreak havoc on the overall pattern.  With a single dipole, impedance and phase are not a big issue.

T

[Steve - K4HX]

when you talk about 50-60feet high do you mean elevation above the closest surface or the earth?

The earth. Unless the roof of your place has a lot of metal in it, the feedpoint impedance of the your dipole will be determine by it's height above ground, not the roof.

[WD8BIL]

Well ifn it's a common stripmall, Steve, the roof structure most likely has a great deal of iron work so I'd expect the feedpoint to be somewhere below 50 ohms. Not low enuf, however, to raise major concerns, as you say.

I'd put the dipole up as intended with the choke as Tom suggested. With the choke you won't need any type of balun.

[KM1H]

It will take a lot more coax than that to be fully effective on 80M and will weigh a lot more than a ferrite sleeve (current balun) balun. You do not want a 1:1 voltage balun.

Carl

[Pete, WA2CWA]

I've used ferrite current type baluns (W2AU or equivalent - high power version) for years on my 160, 80, and 40 meter inverted vee/dipole type antennas. I tried years ago going the route of winding 10 to 15 turns of coax on (I think it was a 4 inch diameter form) a PVC tube and still had RF radiation floating down the shield of the coax. Plus, the coiled coax added a lot of weight and mass to the center support.

[K1JJ]

A balun, long string of beads beads, unun, etc.,  really should be supported in the middle with a support to last the winters and wind.

On 75M I can see your point where 10 turns of coax wound on a 4" form is only about 175 ohms of reactance. If coiled into a hank, it would be more.  But compared to a 50 ohm antenna, I would rather see the reactance at 500 ohms or more.   However, all of my antennas also use a long string of beads over the coaxial feedpoint too. It's easy to do and I figger it can't hurt to have both.

On the higher bands where it takes less coax to give a higher reactance, a coiled coaxial choke is used widely.  Maybe beads are added too, but many of the big guns use them on their Yagi stacks, etc.  I have both techniques  combined on all antennas here and don't have problems with pattern distortion or RF in the shack - signs of feedline radiation.

T

Pic#1:
Here's the driven element of one of the homebrew 40M triple stack Yagis. Notice the combination coiled coax on a 4" PVC form and string of large ferrite beads at the feedpoint. Those ferrites are about 1" diameter and 2" long each.  I figure the overall reactance is at least 900 ohms on 40M.

Pic #2:
Completed stacked 40M Yagis at 63', 125' and 190' all fed in phase. Fixed on Europe.  Other wire Yagis covering west and south.

[K1JJ]

BTW, thinking more about it...

There's one well-known antenna guru who does not recommend using ferrite for antennas because of the possiblity of permanent saturation caused by nearby or near-direct lightning strikes.  He claims coax coiled up is superior and impervious to EMP damage.

I guess the real problem is, how would we even know if our ferrite is damaged? Would it take RF in the shack or something else to alert us?

That is a good reason to use both coiled coax and ferrite in combination - just as an extra redundant system.

Opinions?

T

[Pete, WA2CWA]

BTW, thinking more about it...

There's one well-known antenna guru who does not recommend using ferrite for antennas because of the possiblity of permanent saturation caused by nearby or near-direct lightning strikes.  He claims coax coiled up is superior and impervious to EMP damage.

I guess the real problem is, how would we even know if our ferrite is damaged? Would it take RF in the shack or something else to alert us?

That is a good reason to use both coiled coax and ferrite in combination - just as an extra redundant system.

Opinions?

T

If I had a 40 meter yagi (or any antenna) at 190 feet I might be more concerned about the effects of a nearby or near direct lightening strike. However, given that the typical amateur has his/her wire antennas generally between 30 and 80 feet high in the air strung between trees or poles, I would be more concerned about debris damage (branches falling breaking the wire, poles falling over, etc.). You would have to admit that your antenna installations are not typically what you would find at many amateur locations.

As far as ferrite material being permanently saturated from a lightening strike, I guess you would need to know the effects without using them first and then the effects using them. Gives you a "line in the sand" for measurement and evaluation. I for one thought ferrite material was generally porous, which in your installation, seems to invite water contamination having them totally exposed to the elements.

[KM1H]

How many hanks does it take to make a harvey?

I remember when the TA-33 was popular and recommended a coiled coax that really didnt work very well for TVI. Once the large beads came along it was all fine.

I used to ship those by the hundreds a week for most all antennas before the copy cat balun in a water pipe came along with undersized beads on RG-142 that ran hot at much over 200W. The real scammers used RG-8X.  After that flurry went away I was back to selling the big beads at the same rate again to those who didnt want to spend 2-4 X the price for a QRO balun in a water pipe.

[Pete, WA2CWA]

So, is that ferrite material porous enough to worry about moisture contamination, or did you find that moisture didn't have any effect on the ferrite performance regardless of its material makeup?

[KM1H]

There's one well-known antenna guru who does not recommend using ferrite for antennas because of the possiblity of permanent saturation caused by nearby or near-direct lightning strikes.  He claims coax coiled up is superior and impervious to EMP damage.

Who is this brainiac?

Ive taken direct hits on the 180' tower and also enough spilled over to the copperweld 7 strand 160/80 inverted vees that it melted apart individual strands. There wasnt even a trace of damage to the bunch of 1.5" long by 1" wide beads on the stacked 40M, 4 stack on 20 and the vees that were all on the 180' tower. The shorter towers havent been hit that I know of.

Since the ferrite is not touching any conductor I doubt it is affected.....and neither is the coax at least here.

[KM1H]

They seem to be impervious to water or anything else based on some measurements I made some years ago when the big stacks were removed and cable plus beads salvaged for other projects.

[K1JJ]

Pete, I would think a large, nearby EMP strike would affect an antenna at 190' equally as well as one at 80'.   In fact, my towers rarely get hit - the LOWER trees in the surrounding vicinity take the abuse it seems. Must be the ground radial system bleed off the towers' charge.  Even the guy wire insulators snap when lightning strikes within 1/2 mile away.  One hundred feet seems like a small distance when comparing antennas in this case.

I've never considered ferrite water contamination. I've seen many installations with them exposed to the weather. But it is something to look into for sure.


Carl, the times I talked with Chuck/K1KW his opinion was negative about using ferrite because of potential EMP saturation. At least using it where the ferrite was difficult to service, like on a hard-to-reach Yagi feedpoint. He preferred the coiled coax instead. Though I did see him using some large ferrite donuts on the feedpoint of his 80M rotary Yagi in the 90's when I climbed up there one time. But it was on the center driven element where it could be easily worked on from the tower.  Ask him next time you see him. Maybe he has a different opinion these days, I dunno.


"There wasnt even a trace of damage to the bunch of 1.5" long by 1" wide beads on the stacked 40M, 4 stack on 20 and the vees that were all on the 180' tower."

My question still remains - how would you even know if the ferrite material was permanently saturated if it was?  As Pete sugggested, we need some kind of working baseline, right?  I would think we would need to bring it back onto the bench and run some core permability tests. I'm not trying to be a wise guy, just curious...


T

[W1VD]

NiZn (43,44,51,52,61,67,68 etc) cores are more suseptable to permananet change than MnZn (31,33,46,73,75,76,77,78,79 etc). I've seen both types exhibit irreversible changes, though.

[K5UJ]

pros and cons:

coaxial wound choke:  no loss due to heat; no saturation.  Materials may be already on hand and cheap.   Attention must be paid to construction--you have to get enough turns to get the necessary reactance to have good results.  the maximum reactance comes from a coil where the diameter and length are equal; you have to use coax that can handle your power level and not go under its bend radius with the coil form.  On low bands the coil can add a non-trivial amount to the total feedline length.   Works but over limited frequency range.  Scramble wound chokes are a waste of material.   

Ferrite core baluns:   Small size and work well over wider range of frquencies but can heat up for AM, however make it big and beefy enough and that should not be a problem.  If you saturate and smoke one, you will know about it immediately (been there--may be one of the AM rites of passage hi hi).

Back when I first got on 160 with my inverted L, I thought I needed a choke on the feedline at the feedpoint, so I made a coax choke by winding 100 feet of 213 on a plastic garbage can.  I got a small can that was around 18 inches diameter and wound 30 or 40 turns on it to get a coil that was about 2 feet long.   It was a heck of a 160 m. choke but added 100 feet to the feedline and eventually I discovered that with all my radials I didn't have enough RF on the outside of the feedline to matter so I got rid of it.

Personally I favor balanced feed and a link coupled tuner, a sort of variable balun.  This route can be a difficult one for rotatable antennas, but there are some real interesting photos in the pre-WW2 pre-coax ham literature of quad and yagi installations that rotated with open wire line--very cool to look at but probably a PITA to maintain.  I'm talking about the ones where the feed comes down inside the tower which rotates, to connect with a pair of copper rings standing off from the tower on insulators, one above the other and a few inches out from the legs, and these make contact with a pair of wipers on a post.  The wipers connect to more balanced feedline which goes on to the shack.   I would like to try that.  Very buzzardly.

   

[KM1H]

We should have asked Chuck at Nearfest Tom!
Chuck is no dummy and is a lightning magnet on that ridge but I wonder if it was RF induced having seen a few of his amps in operation

I would think a direct strike would shatter a 43 core and heat would be evident on the coax jacket or vinyl tape.
Its easy enough to test one on the ground with an L meter and a few turns of wire.

If you glue several in series and parallel another group you can build a binocular core transformer for matching an antenna and have complete isolation between the windings. Ive done it at the 1200W level on 6M to feed a 16el collinear, it looked good on a Bird and the antenna worked well. I had no way to measure loss accurately then.

[K1JJ]

Oh, so you were the guy Chuck mentioned about the 6M collinear array. He said you guys sat in front of the computer and designed up a monster array one night...

I think Wayne Green had up a collinear stack too, right?    Chuck said the conclusion was the full wave elements had too much loss being supported on their ends, so the 1/2 wave Yagi was a better idea to stack. He thought the overall loss cancelled the extra ~2.5db of extra gain.

Well, anyway, I took it to heart and designed and built this array of 6M Yagis stacked from 190' down to about 50', all fed in phase.  It was a big job building, climbing and wiring them together. Each Yagi used a homebrew 4:1 coaxial balun, so no problem with ferrites, etc.  I put this array up by myself in two days.  It worked quite well.

If I did it again, I might consider a rotatable H frame, though those are a bear to keep up too.

T

[W2PFY]

I have done the coax balun on 40/80 with 1500 watts RF on CW. I have had no RF issues but part of the reason may be due to the fact that the coax is 275 feet long laying on the forest floor.

[ssbothwell SWL]

holy cow thats a lot of responses! thanks everyone for all the information. sounds like a ferrite sleeve balun would be pretty easy to setup. do i just use whatever torroids have a nice fit over my coaxial cable? how long should i make the sleeve?

will the same sleeve function properly on a variety of bands? eventually i want to get on 160m, but i believe its not allowed on a technician class license.

[KA2DZT]

holy cow thats a lot of responses! thanks everyone for all the information.

In the future that will teach ya to keep the questions simple.

Ask too difficult a question with this gang and you'll get enough information to write a book.

Just put up the best antenna you can for your location.  At only a few watts of power, don't worry about feed line radiation and baluns.

When you upgrade to a JJ level station you can give more thought to all the fine details.

Fred, forum expert on everything important

[Steve - K4HX]

You can buy sleeve balan (often called W2DU) kits. Suggest you go with one of those  to keep it simple at the start. You can always build your own later. The standard kit is good for 80-10 meters. Making it work on 160 is just a matter of adding more toroids. Keep in mind, a coax fed dipole can only be used on one band¹ anyway, so to cover 80-10, you'll need several dipoles or have to go with an open-wire feedline.

1- Actually, a dipole can be used pretty effectively on odd-harmonics (e.g. a 40 meter dipole can also be used on 15 meters), but you may have a higher SWR than on the fundamental.

holy cow thats a lot of responses! thanks everyone for all the information. sounds like a ferrite sleeve balun would be pretty easy to setup. do i just use whatever torroids have a nice fit over my coaxial cable? how long should i make the sleeve?

will the same sleeve function properly on a variety of bands? eventually i want to get on 160m, but i believe its not allowed on a technician class license.

[KM1H]

Oh, so you were the guy Chuck mentioned about the 6M collinear array. He said you guys sat in front of the computer and designed up a monster array one night...

Ah, yes those were religious events   Chuck would come over after work about once a week, we would devour a 6 to  12 pack playing on the PC with YO and AO to come up with some incredible stacked 6M arrays and then get into phasing steering. Some interesting amps got worked out also.

Then we would go to church at a Mexican restaurant, usually in Westford where a few others would join us until the place closed or we got kicked out. It was a weekly AM dinner!

I think Wayne Green had up a collinear stack too, right?    Chuck said the conclusion was the full wave elements had too much loss being supported on their ends, so the 1/2 wave Yagi was a better idea to stack. He thought the overall loss cancelled the extra ~2.5db of extra gain.

Crashcraft built 4 that I know of. One for Wayne, another for Sam Harris, W1FZJ/W1BU, one went to someone in NJ and another somewhere in the midwest.
Waynes wound up with Jack Wilson, W1QXX and eventually to me. Sams never left the Rhododendron Swamp and I tracked down his son Pat, W1HIV (what a call) who knew roughly where it was in the woods and gave it to me.

Both had excellent aluminum but the insulators looked like compressed paper. I tested several plastics for RF properties and settled on 2" black Delrin rod, solid fiberglass rod stock was too expensive. It was fed with #10 copper open wire and the balun mentioned earlier. The SWR curve, bandwidth and performance didnt expose any loss and I even felt the insulators after 10 minutes of a 1000W carrier and there was no heating. I took it down when the 10 and 15 stacks went up on side arm rotators. It just might go on the 180' tower this year along with its twin facing West.

Guys on the East Coast were amazed on how it opened and closed the band and some days Id run well over a hundred Europeans on CW (on E Skip, not F), it was fixed and aimed about right down the northen edge of the Med but worked well from Norway to mid Africa and didnt care where the E cloud was bouncing around. I had a modified Hy-Gain 6 el on another tower at 60' for other directions and to kill some noise. The top was only at 90' and 9' spacing between sections. Two bad features at times was that the F/R was only about 15dB and the forward lobe was barn door wide which made it noisy on receive. OTOH the 8 el yagi which replaced it in place of the Hy-Gain was very quiet as I designed it for a 35-40dB + F/R and the gain was .1dB from theoretical.

If I did it again, I might consider a rotatable H frame, though those are a bear to keep up too.

Im seriously considering a quad of 4 or 5 el to rotate on top of the 180', it should be managable enough and have an excellent pattern; I'll use 2" booms and 7/8 and 3/4 elements. Just took delivery of some rugged Army surplus fiberglass tubing, 48' of 4' lengths that join together, it should make a rugged frame and the 2" x .25"  chrome moly steel mast wont even feel it. I built another 8el CC mod for there but decided it wouldnt survive way out in the open without a major mechanical strengthening. The winds hit the hill upslope, curl up and really blast above the 120' level. All the problems I had with the top 4el 40M KLM was a result and was a huge reason I decided to not put up Chucks 75M yagi at 200' as planned on a former BCB Rohn 65 clone tower.

Carl