melting coils...

[N2DTS]

A few weeks ago, I put up an alpha delta dx lb plus, it does 160 to 10 meters in 100 feet.
It is a fan dipole for 40, 20 and 10 meters, tuning up 15 on the 20 meter part, then uses coils to extend/isolate the 80 and 160 meter parts.
The coils are wound on 1.75 inch plastic pipe, using 18 gauge enameled wire close spaced.
The 40/80 meter one is 69 turns giving .082 mh roughly.

I made some old buzzard transmissions last Sunday, at 3/4 power, about 400 watts carrier, and when I took the antenna down to adjust the swr on 40 meters, I found the coils had got hot, and warmed the plastic pipe up and its about2 inches longer than it was!

I like this antenna because I can jump around all the ham bands without tuning anything, listen remotely on all bands, and I dont need any tuner if I operate where its resonant.
I can also hear people on 160, which my other antenna's did a real poor job with.

So I want to replace the stretched coils and thought I would wind new ones with some 16 gauge enameled wire.
That might not heat up as much as the 18 gauge did.

The questions I have are:
Since the coils isolate 80 from 40 meters, AND load up 80 meters, what effect would the shape of the coils have?
The coil is going to be longer with 16 gauge wire, what if I used 14 gauge wire and it was much longer?

Also, anyone know of anything avalable that is better then plastic pipe?
It takes the antenna strain as it is now, but I suppose I could use an antenna insulator inside the pipe.

Other than the plastic pipe getting soft, things seem to have held up well, I figure 16 gauge coils should reduce the heat some.

Brett
N2DTS

[n4wc]

You might try measuring the inductance of the existing coil.  Use a grid dipper with a know value capacitor.  Then wind a new one with larger diameter wire on a better quality form.   Should be FB.

Bill    N4WC

[WQ9E]

I recall in QST many years ago reporting the author's finding that PVC did not make a good coil form.  His vertical melted with around 500 watts of CW operation.  I would suggest Fiberglass or another suitable plastic for the coil form.

AM and RTTY both do a great job of finding the weak points in your antenna system!  My Desk KW took out a 2KW rated antenna switch in the first week (and it was not running in the 1 KW position at the time).

Rodger WQ9E

[WU2D]

In Boy Scouts we heated sand in a pail on a coleman stove. Then we put a wooden stopper at the end of a PVC pipe and poured the hot sand in. After 20 seconds, we removed the stopper and the PVC was like a noodle. This noodle was quickly put around a form and 1 minute later you had a perfect snowshoe. We made 10 pairs in one afternoon. PVC gets really soft when it gets hot.

http://www.inquiry.net/outdoor/winter/gear/snowshoes/pvc_pipe.htm

Mike WU2D

[N2DTS]

As I said in the original post, the coil measured about .082 MH.
No problem duplicating that, I just wonder what the impact of having a longer coil there will be.
The coils load the antenna on 80, but isolate the rest of the antenna from the 40 meter section.
There is no capacitor, somehow, the coil isolates by itself by inductance.

If I was to make a straping coil, say out of #14, it would be much longer than the one wound with #18, would it still work right, or would the spread out inductance kill the isolation???

I had made (in the past) loading coils, B+W coil stock over a long plastic antenna isolator, with lots of extra room between the isolator and the coil. But the B+W stuff is air wound with spaces, and is now very expensive.

The original coils were wound on grey pipe like what they use for electrical conduit, I assume its some sort of PVC.
I also think the coil got hot and melted the plastic, not RF heating, but who knows...

Yes, AM really causes problems on anything built for ssb!
I think 4 times the power might be ok, if its made for 2kw pep, it might hold up to 500 watts pep on AM (125 watts of carrier). Some stuff holds up better than that, but not most stuff.

The B+W folded all band dipole I tried was actualy smoking at 300 watts AM!
A trap dipole I tried many years ago only went a week under the DX100 abuse before the traps shorted out...

Brett
N2DTS


I would like to find something fiberglass maybe, but where to find some that big and thick...

[Steve - WB3HUZ]

Hint: Dummy loads get hot too.

If they are using the interwinding capacitance to make a trap with that coil, changing the dimensions will most certainly change the operation of the antenna.

[N2DTS]

Yes, I know I am wasting power heating up coils, but I only have 100 feet between trees, like to use all bands, and like to avoid the tuner at least some of the time.
I hope the thicker coil wire will give less loss and heat...

Anyone got a better antenna idea for 100 foot between two trees, at about 40 feet high?

I tried a homebrew G5RV which worked ok on 80 and 40 meters, not at all on 160, needed to be tuned everywhere, and was not as good on receive at least as the alpha delta.

I suppose I could go back to a slightly loaded 80 meter dipole, using big coils to take off 10 feet a side to fit into my 100 feet. I would then have seperate 80 and 40 meter dipoles, and no 160 meter antenna.

I found some fiberglass mast stuff in the basement, I have about 50 feet in 4 foot sections, its slightly larger than the original coil forms, but should work.
I will try making coils out of 16 gauge and maybe 14 gauge enameled wire and see what happens...

Brett
N2DTS

[WA1HZK]

The last time I had a trap antenna working over here Brent, W1IA came over to fire up a couple of 4-400's on slop bucket. After a while the traps burst into flames along with the W2AU Balun. The moral of the story is to build only resonant dipoles and feed them with RG-213 or equal and do not use any connectors outside at the feed point. Direct connections only with the co-ax balun in line with no connectors used. Did I tell you about the no connectors? No PL259's!!! If your running 400 watts your getting close to the dark side and you might as well admit it and upgrade the antennas as they melt into slag. Also, NO TUNERS with internal BALUNS! Use only link coupled tuners if you insist on open wire feed lines. Do not use a tuner on Co-Ax. Your just tuning the line between the transmitter and the tuner. the rest of the feed line is getting hot!
Just venting and saving you the grief of setting your roof on fire.
Thanks Brent!
WA1HZK

[ke7trp]

We could all recomend differnent multiband wire antennas. But not wanting to use a tuner is really going to limit you down to the Alpha or G5RV.  A simple tuner with a Zep or Cobra dipole will be so much better then this antenna. Its known as the dummy load in the sky because of exactly what you had happen. All you can do is wind larger coils and test it out. Maybe you will get lucky. The other option is to just reduce the AM power down to 200 watts. This antenna will handle 200. Running past legal limit on this thing is going to produce the same results you had before on AM mode.

Good luck and let us know how it works out!

Clark

[W2VW]

Welcome to the challenge of making a signal on 160 without living on a farm. Do a search on this forum for a lot of history. "Short 160" "Shortened 160" "small lot" would make a few good word searches here.

In a nutshell, put up as much wire as high as possible centerfed. The wire will have higher than normal antenna current due to low radiation resistance numbers. Use heavy gauge wire. If weight savings is real important wire gauge can be stepped down as you get away from the high current feedpoint. Hanging 20 feet down vertically on both ends will help lessen the loss without ruining your pattern on 40 if you use it there. Number 8 wire is not overkill here for AM QRO. The thing that is melting your existing coils is antenna current.

Resonance of a doublet means nothing. No magic. Look at broadcash antenna. See any resonant ones? The very term resonance is misused by hammy hams.

Open wire feed is a must. It will transform the impedance of your too short antenna to something the antenna coupler can deal with. Feeders can be homebrew using plexiglass spreaders. The local glass shop here whacks the stuff up for a song. Drill holes slightly larger than the wire which passes through. THHN wire works great. Stretch the whole mess out on the ground between some trees and glue the spreaders in place with 2 part epoxy. Makes a nice sandwich, a nice sandwich. Heavy gauge wire is needed here also. Commercial window line will not transfer power into this kind of load with reasonable efficiency. Even the #14 stuff will be too light.

Forget about antenna couplers with 4:1 BalUn. The load we are considering is a low numerical resistive portion of the complex impedance with a good dose of reactance thrown in unless you resonate the feeder length to the antenna. That would take about 75' of feeder on 1885. A 4:1 BalUn is just the wrong thing as it would divide the resistive portion of the complex impedance by four and also have a hard time with the reactance. The AG6K design puts a 1:1 BalUn at the 50 ohm non-reactive part of the circuit where it works best.

Consider a Tom Vu missing link tuner or a AG6K balanced balanced tuner. See AG6K's website or this here forum for the K1JJ link deal. I prefer the AG6K because it is not bandpass and you can swl without retuning. It usually lets one jump to the second harmonic of a band it works on with just a small amount of hocus pocus. Forget the expensive roller inductors shown on the website. Wind your own from refrigerator ice maker supply line. Wind in stereo like a bifilar choke. That makes it easy to unwind one from the other and land up with physically matching inductors. Pay attention to proper coil form factor information shown in the usual handbooks.

I have built several shortened doublets for 160 and am real happy with performance. I melted a lot of parts on the way. One coupler I built for a 124' centerfed wire on 160 showed 5 amps antenna current on the 50 ohm input port and 18 amps in each leg of the feeder on the output. This illustrates just what it takes to transfer the power. Some designs just waste the power and still show great SWR.

Expect very high Q on 160. Your high frequency speech will show a higher SWR than low freq speech! You know right away that the system is lossy if it doesn't. If the frequency of lowest SWR drifts, something is getting hot.

Anyone who can build a receiver like you should have no problem with making a decent signal on 160 with a short wire. Most of the problems can be fixed with extra copper in the right places.


73,
Dave W2VW.

[N2DTS]

Dave, thanks for the great info.

http://www.netcom.army.mil/mars/training/docs/DOCUMENT%20NUMBER%202%20%202007.pdf

I found that site, very interesting, a horz loop with open wire line and the balanced balanced tuner.

A loop would be hard for me to do. I could do a triangle, 100' by 100' by about 40', I wonder how that would work...

I do have access to all sorts of wire, really thick stuff, like #4 on down, lots of scrap at work.
I don't plan on getting on 160 really, although I do like to listen from time to time.

I think I will try putting up an 80 meter dipole, in the trees, made with #10 wire and adjust it so its resonant at 3850.


Brett
N2DTS

[Steve - WB3HUZ]

As Dave said, with 100 feet of space, hanging some wire vertically off the ends will only help. Just 10 feet off each end and you have a full half-wave on 80. I'd hang 20-30 feet, if I was going for 160 meters.

Some ideas at the link below. Have fun.

http://www.amwindow.org/tech/htm/160smallants.htm

[K6IC]

A friend was using a similar-sounding Alpha Delta multi-band DP,  running an Amp on 40 or 80 M.  A neighbor rang the doorbell,  to say that something was on fire in the air,   near a tree.

Upon inspection,  it WAS the Alpha Delta antenna--plastic pipe insulators burned to a crisp.  He sent it back and AD replaced it.   The fellow Ham did have a keen observation,  "These things can be a FIRE HAZARD !' !!    He also noted that some Hams place antennas in attics,  and this antenna construction style might burn down someone's house,  so on.

Nothing against AD,  but  agree that plastic pipe,  while easy to use and inexpensive,  may not be the best coil form.

73  GL  Vic

[KD6VXI]

A friend was using a similar-sounding Alpha Delta multi-band DP,  running an Amp on 40 or 80 M.  A neighbor rang the doorbell,  to say that something was on fire in the air,   near a tree.

Upon inspection,  it WAS the Alpha Delta antenna--plastic pipe insulators burned to a crisp.  He sent it back and AD replaced it.   The fellow Ham did have a keen observation,  "These things can be a FIRE HAZARD !' !!    He also noted that some Hams place antennas in attics,  and this antenna construction style might burn down someone's house,  so on.

Nothing against AD,  but  agree that plastic pipe,  while easy to use and inexpensive,  may not be the best coil form.

73  GL  Vic

I've said it before, and I'll say it again.  PVC is NOT RF Immune.

People argue the point that it heats up in the microwave, so it can't be all that bad on HF.

Yeah, well, in high school, my final project was a 1750 meter TX beacon.

The different colors of PVC would change the inductance of my ant loading coil noticeably.

To each his own, but PVC is NOT RF Immune.  If it heats up in the microwave, don't use it at all for RF.

--Shane

[N2DTS]

I am going to take down the alpha delta and try a full length 80 meter dipole.
I show, for it be resonant at 3850, I should do 60.77 feet each side, how much will having the ends in the trees and maybe bending down change this I wonder...
Maybe I should make it a foot longer? You can always remove wire, adding it is a pain...

I have plenty of #10 and #6 insulated wire.
#6 is very heavy stuff, maybe too much for the trees and rope....

I think I will put the alpha delta up where the 40 meter dipole is, without the 80 and 160 meter parts, so its a 40, 20, 15 and 10 meter fan dipole.

Sounds like a plan.

I should be able to get on 80 and 40 meter AM without a tuner, and low swr.

Brett
N2DTS 

[KB5MD]

I made the mistake of winding a plate choke on pvc pipe once. NOOOOT!  RF can go right thru it.

[ka3zlr]

hey man Yer Antenna's on Fire...holy crap..lol...Have to remember AM is Real Fire in the wire..100% duty cycle...

One time i won a MFG'd g5rv at the hamfest in Washington, I gave it back..one guy said hey man that's a great antenna..I looked at him like great for what...Receiving..LOL..I was not well liked there...LOL....

73
Jack.

[KL7OF]

Some types of PVC plastic pipe seem to work as coil forms...Some do not..I wound a plate choke for my 300-j using 3/4"white PVC (I gave it the microwave test) It has been working FB for years...I have also caught the tree(s) on fire using PVC elements in antennas (Also passed the microwave test)...In the case of the antenna, the coil heated up and caught the PVC on fire..It wasn't the lack of RF immunity on the part of the PVC...  All my antennas are resonant and coax fed now..I am fortunate to have enough room and tall trees to put them up.  I have found that coax fed dipoles usually have to be made slightly shorter than the formula dictates when made from 12 ga THHN and hung at 1/4 wave above ground..I use no connectors, no baluns, and fat coax and they hold up to QRO...

[N2DTS]

I put the 80 meter dipole up today, it was slightly low in freq so I trimmed some off, about 8 inches and it was to short!
I used the #10 insulated wire, and finaly got it resonant at 3870, but for some reason, I cant get the swr lower than 1.5 to 1. I tried adjusting the length of each side in case my measuements were wrong, but changing the length of either side just makes it worse.
Maybe that is an 'in the tree effect' ?
 
Brett
N2DTS

[kc2ifr]

Im sure Ill get flamed for this........but here goes.
The old timers (Im one) always said that if u need an antenna to cover all bands then put up as much wire as u can as high as u can feed it with balanced line and tune it with a GOOD LINK COUPLED ANTENNA TUNER. I have no idea where this BS came from about NO TUNERS......its all BS.
The losses with a tuner amount to crap when u consider that the antenna is a comprise antenna to cover all bands.
If u want to put up with hot coils, hot coax and all the rest so be it.
For  some stupid reason folks seem to think u have to have a resonate antenna for it to work. BULL SH@@.
For what ever reason......some ppl think that an antenna HAS TO BE FED WITH COAX.......again BS.
Again I reiterate we a talking about a comprise antenna.
I use all the above with a 165 ft dipole and it works GREAT..........no magic here.
Good tuner, good balanced feedline, and your on the air.

So there................

Bill   

[Steve - WB3HUZ]

It may never get lower than 1.5:1 and this is perfectly OK. Remember, a dipole over real ground is almost never 50 Ohms (only at one particular height for a given ground conductivity). Modeling shows your 40' high dipole will have a resistance of around 60-65 Ohms. That SWR on 80 meters amounts to, at worst, a mere 0.05 dB additional loss over the coax loss if it were matched to a perfect 50 Ohm load (numbers based on 100 feet of RG213 coax).

Tune your rigs as normal. If you can tune and load the finals to the proper plate current values and you get no arcing (which I doubt you will), you're good to go.

I put the 80 meter dipole up today, it was slightly low in freq so I trimmed some off, about 8 inches and it was to short!
I used the #10 insulated wire, and finaly got it resonant at 3870, but for some reason, I cant get the swr lower than 1.5 to 1. I tried adjusting the length of each side in case my measuements were wrong, but changing the length of either side just makes it worse.
Maybe that is an 'in the tree effect' ?
 
Brett
N2DTS

[N2DTS]

Thanks for the reply Steve.
I think the .5 swr is from the station control setup, if I plug the antenna right into the pro 3, the swr goes all the way down, if I go through the station control, its 1.5 to 1 and changes as I adjust the mod monitor takeoff....

The rig tunes up fine at 3835 and all around 3880 with no tuner.


Brett
N2DTS

[W3SLK]

Let me chime in here with words of wisdom from the antenna Guru himself: K1JJ!
First, my problem was that I could tune FB dipole WITH coax on 1885. However if I went up the band, I was not able to tune out the reactance. This also occurred on 75 meters. I could do FBOM on or about 3705, however it was impractical for me to try it out on 38885 since my VSWR was + 5:1. I am using a Nye Viking MB VI.

Quoting K1JJ:

Hi Mike,

Well, what you are experiencing is the Smith Chart.  The requirement for
more capacitance or more inductance is related to where you are on the
wavelength curve for a given freq.  Ie, Your feedline and flatop are
continually varying from 1/2 to 1/4 multiples of wavelength along their
lengths. You happen to pick a point that is convenient to feed to the shack
like everyone does.   Unless the swr is 1:1, there will be widely varying L
and C reactive components which need matching.   

For example, if the feedline is 1/2 wave multiple long, then L added by the
tuner reflects back L at the dipole. If the feedline is 1/4 wave multiple,
then L reflects back as a C to the dipole.  Anything in between these
lengths become more complex and can mean a combination of L and C.

Simply adding more C to go down in freq doesn't apply when involving long
lengths of feedline. It does work when dealing with circuits having little
or no feedline, like your tank circuit in the rig, etc.

Bottom line is when you are arcing, it's becuz you have created a higher
impedance which means higher voltage across the components.  There is no
cure except to change the feedline length and hope you can match a lower
impedance without seeing large reactive components (and high voltage), or
better yet, build a bigger tuner.   Tuners and coax are somewhat of a pain,
but I understand about 160M and the inability to get a wide ranging flat swr
curve there.  If you want a flat curve, add another set of legs.... One for
1870  and another for 1965 and you will cover the band with < 1.5:1 or so
using coax feed.


The bottom line is unless you are using a perfect 1/2 multiple feedline to
your dipole, what you are seeing for impedance is totally different than
what is really there at the dipole's feedpoint,  requiring consulting the
Smith chart or other formulas.

Hope this helps.

T

Now, to add to what Bill said when he was refering to balance line feeders, Tom said this:

Mike,

If you are seeing a match under 2:1, I would not use the tuner at all for
coax... It just adds loss itself.

Working the other bands with openwire... Well, unless you use openwire, you
can't work the other bands wid that coax ant... :-)  unless you use more
legs.   That's the main substantial reason for openwire.  Otherwise, the
diff between openwire and coax is really nothing, loss-wise on the bands
below 20M, assuming the swr < 3:1 or so for the coax...

But it depends on your ant situation. Do you have room for more single band
coax fed ants? If so, I wud go that route and have instant switching wid no
tuner tuning hassles. That's what I did and now it's a quick click-click for
any band.

If you are limited for room to isolate the ants from each other, then a
multi openwire ant wud be best.   Nothing worse than a smaller ant  stuck in
the field of a larger one to ruin its pattern

I hope this helps out. I like sharing information I have gleaned from others.

[Steve - WB3HUZ]

Good deal. Glad it worked out. Hope to hear you on 80 meters soon.

Thanks for the reply Steve.
I think the .5 swr is from the station control setup, if I plug the antenna right into the pro 3, the swr goes all the way down, if I go through the station control, its 1.5 to 1 and changes as I adjust the mod monitor takeoff....

The rig tunes up fine at 3835 and all around 3880 with no tuner.


Brett
N2DTS

[N2DTS]

Yes, I am well aware that the best performing antenna is one that is as long as possible fed with good open wire line and a good balanced tuner.

The downside of that is you need to tune the antenna any time you change frequency.
You cant even listen unless you tune.
Homebrewing a good balanced tuner is not easy, at least not one that is easy to tune without having to move jumpers and so on. The dual ganged roller inducters looks great, but I dont have any high power roller inductors.


A resonant dipole fed with coax also works fine, as long as the swr on the coax is low, and you stay on lower frequencies where coax loss is low.

I do have 2 antenna's up, one for 40, that I need to replace with the rump alpha delta fan dipole that does 10 to 40, and tune it for the upper part of 40 meters, say 7250, and the new 80 meter antenna is resonant at 3870, a bit high but it will get pulled longer in the next wind storm no doubt, and works with low swr on 3835, 3870 to 3885, which is all I ever operate AM on.

I have about 70 feet of coax on the 80 meter dipole, the RG8M stuff, about 100 feet of the same stuff on the 40 meter dipole.
I could replace some of that with the RG214 I have, the part not hanging from the antenna, but figure the coax connectors would be more trouble and maybe loss than the RG8M.

Brett
N2DTS