HF dipole center support question

[fritz73]

I have an alpha delta DX20 which I am limited to mounting a certain way, as opposed to simply supporting the ends with the center in free space. I have a plumbing vent pipe which appears to be the old cast iron type near the center of the roof, a few feet down from the peak; to which I plan on mounting a vent pipe mount kit with a 5ft mast to support the center of the dipole. The ends will be tied down to eye hooks at the gables of the roof, again a few feet down from the peak due to landlord "concerns". I've read somewhere that the center support should be a non-conductive material, but I'm concerned if 5ft of pvc will hold up in winds. How much would using a standard metal mast in the center affect my SWR on transmit? It seems hard to find a solid answer to this question.

 

[n4yek]

I have a friend that uses his ham radio tower to support his dipole. He seems to have no problems at all. I say go for it, if it don't work then you will know.
Schedual 80 pipe is stout, and at 5 feet I don't believe you would have to worry about it.

 

[kb2vxa]

The center support seems fine to me but my concern is how you get the ends down to the gables without touching the edge of the roof or the overhangs.

 

[k9rzz]

How much would using a standard metal mast in the center affect my SWR on transmit?

As long as the antenna is insulated from the metal, you're golden. Keep the wire from laying on the roof and you should be in fine shape!

 

[fritz73]

KB2VXA: I'm thinking of using some rubber fuel line to slip over the ends of the rope to protect it if it comes into contact with the edges of the roof. K9RZZ: It was on the Alpha Delta website where I read the center insulator had to be at least 18" away from any type of metal support to prevent detuning. This is what raised my concern. Thanks very much for the help guys- KC9ETG

 

[N1BHH]

That Alpha Delta antenna is made with jacketed wire and has end insulators. Just mount a small pulley with light duty rope to a mast and pull the center up. Then take the ends and pull them out in whatever direction you want them, with some more light duty rope. It's cheap enough at Walmart to get short lengths to use.

 

[k9rzz]

KB2VXA: I'm thinking of using some rubber fuel line to slip over the ends of the rope to protect it if it comes into contact with the edges of the roof. K9RZZ: It was on the Alpha Delta website where I read the center insulator had to be at least 18" away from any type of metal support to prevent detuning. This is what raised my concern. Thanks very much for the help guys- KC9ETG

Well, if that's what the manufacturer tells 'ya, I'd guess I'd listen to 'em. Don't be afraid to stray a little and experiment though.

 

[kb2vxa]

Putting the rope or even coax (as I have) through Neoprene hose to prevent abrasion is a capital idea. I somewhat disagree with Alpha Delta since I have hung dipoles with the center insulator or balun closer with no ill effects. The idea is to provide some spacing, a few inches will do and if it detunes somewhat no big deal, very few antennas don't need a bit of "fine pruning".

 

[fritz73]

To cut or to tune?

Well I have the antenna up and connected to my Kenwood TS440SAT. Overall SW reception seems good. Problem: I bought the DX20 because it is the longest one I can go with which would be good for reception, but I planned on using it for TX on 10mtrs. My built in tuner is NOT happy with it on any freq above 20mtrs. I seem to remember reading somewhere this is because each leg of the dipole is a halfwave for 10 which is NOT good apparently if you want to use it for TX on that band. Is this correct? remember it is coax fed not ladderline and I want to stay with coax for various reasons. The built in tuner has a very limited range, would it match up if I got an outboard wider range auto tuner like the LDG Z11pro? I would like to know before I waste money on a tuner. I know I could cut the antenna shorter for 10 instead of 20 but I am afraid of how much this will affect my overall reception strength for SW as I enjoy SWL and scan the bands a lot. I also do not have a ground hooked up to the radio yet as I understand dipoles do not normally require a ground, but would this make a difference also? Luckily I have no problems with EMI on RX without it.

 

[kb2vxa]

Oh boy that was a long pause and you're back again with another question. (;->) OK, I'm The Answer Man so here it is. A simple dipole is a half wave on it's fundamental frequency and the feed point impedance is roughly 70 ohms (close enough to 50 for government work) and a 1:1 current balun helps tremendously to decouple it from the transmission line thus mitigating radiation from the coax.

Now you want to use it on the higher frequency bands but you forgot that feed point impedance so now on some you have a ridiculously high SWR. Take a closer look, you get a good match on the fundamental frequency and also on odd harmonics, that is times 3, and times 6. You have a bad mach on even number harmonics because now the coax is looking at voltage rather than current. Current is low impedance, voltage is high impedance so where there is high voltage and low current you have a bad mismatch.

Back on the other end of the coax a "tuner" more properly as Johnson called it a match box that high impedance may be transformed down to 50 ohms to make your transmitter happy but there is a fly in the ointment. It does absolutely nothing to change the actual SWR and high SWR means high return losses so all you're doing is satisfying the transmitter but now your coax is soaking up energy like a sponge. One other thing, your balun is doing likewise and complaining mightily, if it arcs or overheats it's done for so basically you're restricted to 100W or less, amplifiers are not allowed.

If that makes you unhappy get rid of the balun and coax, use open wire feed line commonly called ladder line. You can save some money by buying common 16ga stranded, insulated wire and enough 3 or 4" ceramic spacers to do the job, the insulation on the wire is superfluous but a bugger to strip off so don't bother. Notice I don't recommend twin lead or window lead often called ladder line but it's not, that stuff develops some nasty loss when wet. (Don't argue fellers, it's been proven with accurate test measurements taken by engineering professionals.) Open wire ladder line cares naught for SWR, it has very little loss (far less than coax) either way. Just don't run it across a roof lying flat or closer than about a foot to metal, it hates that.

Now back to the tuner, I much prefer one with balanced output, they take far more abuse than those with internal output baluns for the same reason as previously stated, they tend to fry when presented with an improper load. I nearly destroyed a 3KW tuner with only 120W of AM under an unusual condition, high voltage across the output terminals. It just so happened that the dipole was full wave at that particular frequency, high voltage at the feed point. The open wire feeder just happened to be a half wave so what it saw at the antenna the tuner saw at the other end, nearly fried the balun with a fiery RF arc. That should give you a hint, avoid half waves (even number harmonics) on the line at any frequency!

Lastly as I finish writing this book there is the fan dipole. That design avoids all that mess when you take the fundamental frequencies and third harmonics into account. For example if you cut the elements for half waves on 40M, 20M and 10M you also have the 40M elements operating on the third harmonic for 15M so by all rights you can feed it with coax and don't need a tuner. The WARC bands tend to complicate things and this is getting a bit long, I'll leave the rest to your imagination provided you follow the clue trail and don't ask me again. (;->)

I leave you with this thought, an old ARRL Radio Amateur's Handbook from the 50s or 60s will have an excellent chapter on such antennas. The beauty of the beast is it has the best graphics I've seen anywhere to help you understand voltage and current distribution vs. wavelength along a conductor such as an antenna element.

Doggone it, I forgot all about your ground question! Dipoles being balanced antennas do not need operating ground but safety ground is helpful. Otherwise there is static charge and lightning protection to consider but that horse has been beaten to dust so I leave it to you to take your life in your own hands. Should you or any member of your antenna team be captured or killed this ham will disavow any knowledge of your actions. Good luck on your mission Mr. Phelps, this thread will self destruct in 5 seconds.

Goodbye and good luck, I'll see you on the dark side of the moon.

 

[prcguy]

By quirk or accident, I mostly agree with what Warren said here. Don't worry about using a metal mast with coax, the antenna is mostly at a right angle to the mast and it will not influence anything.
prcguy

Oh boy that was a long pause and you're back again with another question. (;->) OK, I'm The Answer Man so here it is. A simple dipole is a half wave on it's fundamental frequency and the feed point impedance is roughly 70 ohms (close enough to 50 for government work) and a 1:1 current balun helps tremendously to decouple it from the transmission line thus mitigating radiation from the coax.

Now you want to use it on the higher frequency bands but you forgot that feed point impedance so now on some you have a ridiculously high SWR. Take a closer look, you get a good match on the fundamental frequency and also on odd harmonics, that is times 3, and times 6. You have a bad mach on even number harmonics because now the coax is looking at voltage rather than current. Current is low impedance, voltage is high impedance so where there is high voltage and low current you have a bad mismatch.

Back on the other end of the coax a "tuner" more properly as Johnson called it a match box that high impedance may be transformed down to 50 ohms to make your transmitter happy but there is a fly in the ointment. It does absolutely nothing to change the actual SWR and high SWR means high return losses so all you're doing is satisfying the transmitter but now your coax is soaking up energy like a sponge. One other thing, your balun is doing likewise and complaining mightily, if it arcs or overheats it's done for so basically you're restricted to 100W or less, amplifiers are not allowed.

If that makes you unhappy get rid of the balun and coax, use open wire feed line commonly called ladder line. You can save some money by buying common 16ga stranded, insulated wire and enough 3 or 4" ceramic spacers to do the job, the insulation on the wire is superfluous but a bugger to strip off so don't bother. Notice I don't recommend twin lead or window lead often called ladder line but it's not, that stuff develops some nasty loss when wet. (Don't argue fellers, it's been proven with accurate test measurements taken by engineering professionals.) Open wire ladder line cares naught for SWR, it has very little loss (far less than coax) either way. Just don't run it across a roof lying flat or closer than about a foot to metal, it hates that.

Now back to the tuner, I much prefer one with balanced output, they take far more abuse than those with internal output baluns for the same reason as previously stated, they tend to fry when presented with an improper load. I nearly destroyed a 3KW tuner with only 120W of AM under an unusual condition, high voltage across the output terminals. It just so happened that the dipole was full wave at that particular frequency, high voltage at the feed point. The open wire feeder just happened to be a half wave so what it saw at the antenna the tuner saw at the other end, nearly fried the balun with a fiery RF arc. That should give you a hint, avoid half waves (even number harmonics) on the line at any frequency!

Lastly as I finish writing this book there is the fan dipole. That design avoids all that mess when you take the fundamental frequencies and third harmonics into account. For example if you cut the elements for half waves on 40M, 20M and 10M you also have the 40M elements operating on the third harmonic for 15M so by all rights you can feed it with coax and don't need a tuner. The WARC bands tend to complicate things and this is getting a bit long, I'll leave the rest to your imagination provided you follow the clue trail and don't ask me again. (;->)

I leave you with this thought, an old ARRL Radio Amateur's Handbook from the 50s or 60s will have an excellent chapter on such antennas. The beauty of the beast is it has the best graphics I've seen anywhere to help you understand voltage and current distribution vs. wavelength along a conductor such as an antenna element.

Doggone it, I forgot all about your ground question! Dipoles being balanced antennas do not need operating ground but safety ground is helpful. Otherwise there is static charge and lightning protection to consider but that horse has been beaten to dust so I leave it to you to take your life in your own hands. Should you or any member of your antenna team be captured or killed this ham will disavow any knowledge of your actions. Good luck on your mission Mr. Phelps, this thread will self destruct in 5 seconds.

Goodbye and good luck, I'll see you on the dark side of the moon.

 

[knightrider]

I have a fan dipole that works on 10-15-40m, and without a tuner. It may just be luck, but it works fine. It was something I had built when I was a kid for SWL, and decided to try it again some 40 years later for transmitting. A few changes were made, in that I built this one to cover the above bands, but the SWL version was designed for 10-80. On this antenna I have the 40m dipole as the main support wire, and the 10m as the "fan", in that it is angled down from the main wire. It is up about 25', and 40m is horizontal. It is about 10' above a metal roof, and a small part of the 40m leg does go into ree limbs/leaves. The 40m section also works for 15m. My SWR is at 1.5:1 or less on all 3 bands (CW part of 40 & 15m, and CW/Voice of 10m). It works very good on recieve for 20 and 80 as well. I have a homebrew coax balun, and it is fed with RG-8X. Sometimes a blind hog will find an acorn, and maybe that is where i stand...
73,
Paul Munsel N5XMV

 

[fritz73]

update

Well, I took down the DX20 and put up a multiband- The DX-EE based on all the good reviews on Eham. It does seem "pretuned" to the low ends of the bands as the reviews state, but at least my built in tuner seems to like it better which is definately a step up! Being a "multiband" though I notice how it is somewhat narrowbanded on the bands its designed for, so I also got the LDG z11pro autotuner. (I'm a little hesitant on trimming this antenna as I've read mixed results and Alpha Delta doesn't seem to recommend it.) Anyway, I hooked up a CB for testing and everything seems so far so good. I have the tuner grounded to a cold water pipe as recomended. Only having a tech license I've yet to get any replies on 10 meters(only HF I can TX on) which is to be expected I guess since I get nothing there but static while all the other bands are hopping like crazy. It would be nice to hear someone out there to contact if only for testing/adjustment purposes! When they say dead they really mean it! Oh well, study for my General and back to RX only for a while I guess. 73