Shortened 160m antenna Idea

[Ed/KB1HYS]

Was reading through an old Handbook.
In the section on shortened HF antennas,
showed a Helically wound vertical,   basically a Halfwave of wire wound around a form with the feed end at the ground with a radial system.

Well I don't have space for a big radial system so---
I was thinking of making two sections of 1.5" PVC 10' long(each) and connecting them ala Dipole fashion and hanging up vertically in a tree. 
Run the Coax down the inside of the pipe, keeping the portion connected to the shield on the lower end, where the coax passes through.
A big "stinger" antenna.

Yea I know, lossy, inefficient etc.  But If it puts out, well just thinking out loud.
If I had the horizontal Space I'd mount it horizontally and maybe rotatable....

[K1JJ]

Hi Ed,

Yes, it will work.

The reason for any loading coils on an antenna are mainly to bring the input impedance up at the center feedpoint for the use of coax.  As you probably know, coils don't add anything to the radiation pattern... they act just like a straight wire in that respect. In fact, they add loss to the system vs: having a staight wire there, except for the fact that they keep the lowest current/ low impedance point higher than it normally wud be with just a short wire, wavelength-wise. ie, Extreme low impedance = more loss.

As an alternative, if you did the same config with good quality and heavy  openwire feeders using the same antenna with only heavy wire, minus the coils, the performance in all respects would be about the same.  But coax is more convenient, so may be worth the effort.

T

[W3SLK]

Here's another idea. Last night I was in a QSO with a fellow using a discone antenna from 75~20 meters. I forget how he loaded it but he loaded into the bottom part of the discone on 160 meters. He was using a Multi-Elmac into an AL-1200 amp. His QTH was about 60 miles NW of Grand Rapids, MI, (right across Lake Michigan from Miluakee). He was between Guess 5~8 on the HQ-140-X's guess meter. I thought it was doing fine business considering the distance. May be worth trying. I'll have to look in my log book for the name and call sign.

[N5RLR]

Was reading through an old Handbook.  In the section on shortened HF antennas, showed a Helically wound vertical,   basically a Halfwave of wire wound around a form with the feed end at the ground with a radial system.

Well I don't have space for a big radial system so---
I was thinking of making two sections of 1.5" PVC 10' long(each) and connecting them ala Dipole fashion and hanging up vertically in a tree.  Run the Coax down the inside of the pipe, keeping the portion connected to the shield on the lower end, where the coax passes through.  A big "stinger" antenna.

Yea I know, lossy, inefficient etc.  But If it puts out, well just thinking out loud.
If I had the horizontal Space I'd mount it horizontally and maybe rotatable....

Hmm, this got me to thinking, as I’ve also contemplated putting up helical antennas.

According to info I’ve found, 1.5" PVC works out to be 1.99" actual OD.  The circumference would thus be 1.99 * 3.1416 = 6.25". Assuming this to be the circumference of a coil turn [in reality, a coil turn isn’t going to be a perfect circle, we can use this for discussion's sake] on your proposed antenna, this looks as if it’d be:

Halfwave @ 160m.........................5616 / 1.9 MHz = 2,955.79"
Turns Total..................................2,955.79" / 6.25" = 472.93 [round to 473]
Turns / Foot for 10' form [pipe]......47.3, or about 4 per inch

Now, am I thinking outside the box, or should I crawl back under it?

[Ed/KB1HYS]

According to info I’ve found, 1.5" PVC works out to be 1.99" actual OD.  The circumference would thus be 1.99 * 3.1416 = 6.25". Assuming this to be the circumference of a coil turn [in reality, a coil turn isn’t going to be a perfect circle, we can use this for discussion's sake] on your proposed antenna, this looks as if it’d be:

Halfwave @ 160m.........................5616 / 1.9 MHz = 2,955.79"
Turns Total..................................2,955.79" / 6.25" = 472.93 [round to 473]
Turns / Foot for 10' form [pipe]......47.3, or about 4 per inch

Now, am I thinking outside the box, or should I crawl back under it?

Stay out of the box for a while...
That sounds about right. I've got a 500' of 12 gauge wire so   I was initially considering putting up just a Vertical ground mounted type in the Garden, but problems with runing any kind of decent radial system, worries about the proximity too the house letting RF into various things, and the XYL's reaction to my request to put up an antenna in HER garden...    So I figured if you can use a helically loaded section to be the radiator, why not use the same for an opposite side and make a dipole.  Suspending such a thing horizontally between my trees would work, but It will be higher and easier to just haul it up flagpole style./ Besides at the height I'll have it at I don't think there'll be a lot of difference in the pattern if its horizontal or vertical. Supposedly a Helical antenna is a bit noise resistant, as it is fairly narrow band.
 Well hope to get it up by Sunday evening. We'll see!! 

[KB2WIG]

sounds like its worth trying......   When you feed it with coax, are you snaking the coax up the bottom helix to the 'dipole' part??? I'm wondering how you decouple the feedline( if you do )?? Or  AM I totally fubared as to what ur trying to do?   I'm thinking a modified counterpoise system..... klc

[WA1GFZ]

I would think 3 or 4 inch would allow space between turns or larger wire.

[Ed/KB1HYS]

sounds like its worth trying......   When you feed it with coax, are you snaking the coax up the bottom helix to the 'dipole' part??? I'm wondering how you decouple the feedline( if you do )?? Or  AM I totally fubared as to what ur trying to do?   I'm thinking a modified counterpoise system..... klc

Well the plan is to feed the coax up through the lower section, and connect the shield to the lower element, I figure that since the coax shield and the adjacent antenna element are connected effects will be minimal.   IF I went horizontal I'd just try to pull it away at 90 degrees as usual. 
I am wondering if its worth it to wind some kind of balun for the feed point. 


 

I would think 3 or 4 inch would allow space between turns or larger wire.

Yes, and I think a larger Diameter would provide a little more bandwidth and be a bit more efficient(?), it would definitly be a go if I was doing the Vertical (ground mounted) Version.  BUT the weight would start to get prohibitive,  using PVC pipe anyway.  an additional plus is the smaller diameter is a bit more stealthy. The XYL takes a dim view of my Copper Spiderwebs...  (her term)   

[Fred k2dx]

I've built some helically loaded verticals, using pvc and old fibreglass stationmaster tubes. They work. It's all been published, there are trade offs in length/diameter if you want to split hairs. Be aware that the halfwave of #12 will get heavy on the tube. I have also found copper tape works very well and is much lighter. It is available in different widths, I've used 1/4" and soldered the joints. Then wrapped the thing in a layer of Scotch electrical tape. Works well and is much easier than trying to deal with turns of wire. It's much lighter and should carry substantial current due skin effect. Be sure to have a whip on top. See the old ARRL handbooks, they used to show a 40 meter version.

[K1JJ]

I have also found copper tape works very well and is much lighter. It is available in different widths, I've used 1/4" and soldered the joints. Then wrapped the thing in a layer of Scotch electrical tape. Works well and is much easier than trying to deal with turns of wire.

Hmmmm.. that's an interesting idea, using coper tape.

I plan to make up four big coils for a 2el 75M rotary Yagi this Spring and might use  4" diameter ABS plasdick pipe as forms. I would use a 1" solid fiberglass rod as the aluminum element buffer sleeve and attach the ABS pipe to this fiberglass rod.

The copper tape would make it much lighter. Since two coils are used for each el and each out about 24' from the center, the current is not so bad as compared to at the middle of the el at current point.

I'll have to think about this some more.

T

[Fred k2dx]

Copper tape is an efficient RF conductor due to skin effect. RF is conducted on the outside of a conductor and surface area is what counts. Think of an antenna element made of solid rod, it will work but be heavy. The center is of no use. Or a heavy coil wound of copper tubing as opposed to solid copper.

A source of 1/4" wide tape (not sure if it's enough surface area for high power 160 meters) is available at hobby places and is used for making stained glass windows. I've seen wider 1/2" copper tape used for EMI bonding but don't know where to get it. The 1/4" is fine for several hundred watts.

SGC mobile HF whip antennas are wound with copper tape. They are covered in shrink plastic.

[Warren]

Ed,

    You will have a very high voltage at the end of the helical antenna (think Tesla coil). Be sure to put a whip or disc at the end of the coil to dissipate any corona. Your antenna will be very narrowbanded as well, 1  kHz to maybe 2 kHz bandwidth max! Expect the swr to rise with modulation (unless you use a narrowband mode like cw or psk 31).
     There is an interesting program (freeware) called Helical3 at Reg Edward's G4FGQ's site:
http://www.btinternet.com/~g4fgq.regp/page3.html#S301%22

73 & GL
Warren K2ORS/WD2XGJ

[Steve - WB3HUZ]

I've made numerous antennas (not helical wound types) with copper tape. It works well.

[Warren]

Well I don't have space for a big radial system so---
I was thinking of making two sections of 1.5" PVC 10' long(each) and connecting them ala Dipole fashion and hanging up vertically in a tree. 
Run the Coax down the inside of the pipe, keeping the portion connected to the shield on the lower end, where the coax passes through.
Yea I know, lossy, inefficient etc.  But If it puts out, well just thinking out loud.
If I had the horizontal Space I'd mount it horizontally and maybe rotatable....

  Ed,
       A couple of more points. The antenna will be so small (and close to the ground) that it will look like a point source - no directivity. No point in rotating it. Also, I think that you would be better off with a single larger helix and put in the best ground system you can get away with rather than trying to make a dipole out of two smaller helixes. Also keep in mind that due to the very high impedance at the end of the antenna, it will de-tune if there is any precip (or even humidity).
73 Warren K2ORS/WD2XGJ

[K1JJ]

I've made numerous antennas (not helical wound types) with copper tape. It works well.

Hi Huz!

Yep, tape is used by many antennas these days.

I was just thinking of what the equiv tubing size would be for a flat piece of copper tape.

If you were to take a 1/2" wide length of copper tape and roll it into a tube, I figure it will be slightly larger than a 1/8" diameter piece of tubing... pretty small diameter, actually.

1/8" X 3.14 [pi] = .3925" diameter.

So, 1" wide tape is roughly = 3/8" copper tubing for pure surface area, allowing for skin effect.

The area taken up by 1" tape is pretty wide, thus the desired 1:1 or 2:1 form ratio of a coil would be hard to achieve unless the coil diameter is much bigger than with using tubing.  Or, with wide tape, the coil will end up too long vs: diameter, making the Q poorer.

No free lunches, I guess.  For 160M  or 75M coils, physical size and weight is a factor if supported in a horizontal span.  

When modeling 30 ohm input LOADED 75M Yagis, I've seen the efficiency drop from 92% down to 82% just by small changes in coil Q, coil tubing diameter and form factor. So, it can be worthwhile to get a high Q coil for low impedance, heavily loaded systems.

If I had an electrical choice, I would use tubing for a better final physical form factor vs: tape, but then there is weight and support needed that may make tape a better choice for the particular situation.


T

[WA1GFZ]

Tubing would take heat better than tape for those QRO nights with ice on the antenna.  One could buy a roll of copper flashing which is a lot thicker and saw the desired width. I think it comes about .02 inches thick.

[Warren]

Tubing would take heat better than tape for those QRO nights with ice on the antenna.  One could buy a roll of copper flashing which is a lot thicker and saw the desired width. I think it comes about .02 inches thick.

Frank,
    If you have ice on the antenna you will be seriously de-tuned! You will have a hard time putting any power into it.
    I have a 160M 8' helical mobile antenna (made by Mobile Mark) - it detuned very easily when the wx changed.  If you want to put any power into it you will need a capacity hat at the end or the PVC (tube or insulation on wire) will catch fire for sure!
    Another idea, you should taper the pitch of the winding, turns further apart at the base and closer together at the top. This will give you a better (more constant) current distribution in the antenna.
You can match the antenna by taping the helix a few turns up from the bottom.
    I did make some contacts with the 160M helical antenna but was pretty pw compared to the bug-catcher. The swr shot up with modulation due to the narrow bandwidth of the antenna. Also tried running an amp - 100 watts carrier out and I had SERIOUS corona at the end of the antenna.
73 Warren K2ORS/WD2XGJ

[KB2WIG]

I like the idea of the 'dipole' attempt....  gud experimentation...  the Testla eliminators are an area of research... I wonder if some 26 Ga spider web at the end might help... you can always stick some lime to the corona.............

[Warren]

Ed,
     You might want to do some reading on mobile antennas and how to optimize them. I firmly believe that you would be better with a single larger helix than 2 small ones as a 'dipole'.  Take a look at W8JI's excellent treatment of 160m mobile antennas:
http://www.w8ji.com/mobile_and_loaded_antenna.htm

73 Warren K2ORS/WD2XGJ

[Ed/KB1HYS]

Good suggestions.  If I can get away with using a limited, crappy ground plane and have better performance than trying to produce a dipole type helix I guess I'll go that route.

I was planning on putting a whip on each end to help deal with the corona problems.  If I go with the vertical ground plane I will use a capacity hat.  Probably make one using the top 1/4 or so of the guys bonded together and Isolated from the remaineder with egg insulators or some such.

Thanks again for the suggestions... keep em coming!!

[Warren]

Ed,
     A couple more thoughts. Though copper tape or tubing might be a better conductor, it would be difficult to get a long enough piece - you need almost 500'. Think of the hassles of trying to splice together ten 50' pieces of tubing or whatever.
     Another idea for a limited space antenna, a magnetic loop antenna made up of copper pipe and resonated with a capacitor. Doesn't rely on a ground connection so you are more in control of the efficiency. You need a high-Q, high voltage capacitor (either a vacuum variable or a homebrew cap). It's less affected by weather or its surroundings and its easier to QSY by changing the capacitor setting.
    A couple of references for you:
http://www.standpipe.com/w2bri/index.htm
http://home.datacomm.ch/hb9abx/loop1-e.htm
http://www.qsl.net/mnqrp/Loop/Mag_Loops.htm
http://www.aa5tb.com/loop.html

73 Warren
K2ORS/WD2XGJ

[Steve W8TOW]

For 160m/40m I have been playing with a coax fed "loaded" dipole for about a year now.
On 40m the ant works about like any 40m 1/2 wave ant...on 160m it performs about only slightly less
than I expect a full size 1/2 wave  horiz ant to do.
The loading coils are about 33' out from the feedpoint, plus about 25' added to each end past the
coils.
This spring, I plan to relocate the ant,& feed it with 600 ohm open wire line, to a remote "Johnson" style mbox.
will see how that works.
73 steve
8tow

[wa2zdy]

As a 13 year old JN (and one full of himself at that) I read in the 1969 Handbook that a half wavelength of wire helically wound will act as an electrical quarter wave.

I took one of those fibreglass flag rods (remember the orange flags folks had on their bikes in the mid 70s?   Yeah, one of them) and wound about 65 feet of insulated around it.  Close wound it took up the etire length of the pole.   Then I stuck it on my ten speed with ideas of bicycle mobile with an HW7. 

The old guy who got me started hamming tended to humour me but was willing to show me that this thing would not work.  We parked my bike outside his shack and ran coax to it, sheild to the frame of th bike.

The SWR wasn't extreme, maybe 2:1 in the 40 phone band and he fired it up and made contacts with it.  Obviously he didn't do as well as his dipole in the trees, but his buzzardly old cronies heard him and he was able to make solid contacts with the bike antenna.

So the theory is there.   I've got a pair of 8 foot fibreglass rods I'd bought to experiment with similar antennas, but since I no longer drive, that idea has been pushed aside.  Now though you've got me thinking.  I doubt such a short helical antenna will work any better than the linear loaded dipole I have up in the attic, but who knows what I might learn trying.

[Ed/KB1HYS]

well, I've got half this thing built...  seems like there's always something that interferes with working on the radio, antennas etc.

Wrapped 495 turns around a 10 ft pc of pvc pipe... Good work out for the wrists!!!!  at least if this doesn't work out I'll have plenty of wire to make up a BIG dipole...

[Fred k2dx]

It makes for easy winding to mount the tube so it rotates as you apply the wire or tape.