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Author Topic: Coax Vertical for 2m question  (Read 812 times)
WA3WZR
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Posts: 4




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« on: September 25, 2009, 07:21:24 AM »

Having a need for a 2 meter antenna and having some spare coax laying around, I decided to experiment and build a coax vertical.  I have the SWR at a reasonable level for 2m on both an Antenna analyzer and on an SWR meter (using my HT as a transmitter).

When I add coax to it, to about the length I would use when I mount it, the SWR goes off the scale.  I have tried various lengths of coax with the same result.

I'm puzzled.
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WA7NCL
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Posts: 625




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« Reply #1 on: September 25, 2009, 07:34:30 AM »

got any radials on it or ground plane?

if its a 1/4 wave vertical it needs radials or a ground plane.

If you are trying to built a vertical sleeve dipole out of coax, you will have a lot of problems with antenna coupling to the outside of the transmission line.  This means the transmission line is part of the antenna.  You end up with a 2m long wire pointed into the sky.

There are many ways to decouple the end of a sleeve dipole.  They involve various forms of chokes.

Search the net or get an antenna book.
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WB6BYU
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Posts: 13335




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« Reply #2 on: September 25, 2009, 09:40:46 AM »

This would be easier for us to evaluate if you described
what short of coax vertical you were trying to build.
I can think of several very different types that would
fit that name.

The most likely problem is that the shield of the feedline
is acting as part of the antenna.  You'll either have to
change the design or use some sort of decoupling, which
might be a set of radials (or two sets), a choke of some
sort, etc.

Decoupling the coax shield from the antenna is one of the
more common problems with elevated vertical antennas.
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WB2WIK
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Posts: 20611




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« Reply #3 on: September 25, 2009, 10:15:16 AM »

If you check this link:

http://books.google.com/books?id=f3asJreno0YC&pg=PA346&lpg=PA346&dq=coaxial+vertical+antenna&source=bl&ots=06bsbcmCJQ&sig=2jnBpTFbwXe2NFqF4pm7iirsLhg&hl=en&ei=TPi8SrKdL47StAPJjrDcBQ&sa=X&oi=book_result&ct=result&resnum=9#v=onepage&q=coaxial%20vertical%20antenna&f=false

Look at Figure 18-5B to see how a "real" coaxial vertical antenna is generally made.

In lieu of the materials shown, the whole antenna can be made of coaxial cable without tubing, etc; however you must be very careful how you make it.  I've found the easiest thing to do to make the lower "sleeve" element is to:

1.  Expose 19" or 19-1/2" of center conductor and dielectric of the coax as the upper radiating element.

2.  Trim 1/2" or 1" of outer jacket material from the coax just below that, i.e., 19 or 19-1/2" down from the "tip" of the exposed center conductor.

3.  Slide a piece of brass or copper tubing 19" long to make a fairly snug fit over the coaxial cable jacket (e.g., for RG-213/U, use 1/2" ID brass or copper thinwall tubing).  Solder the coax outer conductor exposed in step 2 to the "top" of this tubing, leaving the rest of the tubing covering 19" of coax below that point.

4.  Securely tape that connection made in step 3; add a couple of nylon tie-wraps secured to the coaxial cable right at the "bottom" end of the brass or copper tubing, as a mechanical restraint.

5.  Attach a string or lightweight rope to the very top of the upper radiating element created in step 1, so the antenna can be "hung" from a tree limb or some suitable overhead support.

6.  Any length of coax below the tubing "sleeve" should not impact the way this antenna works; it's pretty well decoupled.  However if you find there is still coupling between the coax and the antenna sleeve, you can add 4-5 snug fitting ferrite cores over the coax right at the bottom of the sleeve to further decouple the transmission line from the antenna.

Cost of this is just the coax itself plus 19" of hobby tubing (maybe a couple of dollars).

WB2WIK/6
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WB6BYU
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Posts: 13335




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« Reply #4 on: September 25, 2009, 11:44:22 AM »

This all presumes that there isn't some mechanical
problem when you add the extra coax, such as shorting out
the coax, etc.

I've built a number of coaxial dipoles where the shield
of the coax is folded back over the standing part of the
coax.  Yes, they can work, but the SWR usually varies a
lot with handling due to the RF on the coax.

But generally the SWR won't go over 3 : 1 or so due to
RF on the shield.  If you are seeing 10 : 1 or worse,
that sounds like a short or open circuit problem, or
something else fairly major (and, hopefully, easy to find.)


One other problem with the common implementation of a
coaxial dipole is the voltage difference between the
bottom of the dipole element and the coax shield right
inside it.  There is a strong RF field across the
coax outer insulation, which isn't necessarily designed
for good RF performance (unlike the center conductor
insulation.)  So the losses may be higher.  You can get
around this by using a larger diameter tube for the
lower half of the antenna with spacers to keep the coax
near the center of it.  Or taper the lower tube so it is
further away from the coax at the bottom.  Eventually
this becomes a set of sloping radials...  

In fact, the last time I needed a quick and dirty antenna
I built a wire ground plane with two radials that were
held apart by a stick, and it was much easier to tame
than the coaxial dipoles I've built.  (The SWR was better,
too.)
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WA3WZR
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Posts: 4




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« Reply #5 on: September 25, 2009, 12:33:12 PM »

I built it essentially the way WB2WIK describes it in his post, except I don't have a copper sleeve, just using the coax shield.

WA3WZR
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WB2WIK
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Posts: 20611




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« Reply #6 on: September 25, 2009, 12:53:26 PM »

>RE: Coax Vertical for 2m question       Reply
by WA3WZR on September 25, 2009    Mail this to a friend!
I built it essentially the way WB2WIK describes it in his post, except I don't have a copper sleeve, just using the coax shield.

WA3WZR<

::Does that mean you peeled the shield back 19" so it would wrap over the outer jacket of the cable below the radiator?
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WB6BYU
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Posts: 13335




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« Reply #7 on: September 25, 2009, 01:13:41 PM »

I've done this two ways, either by bunching up the braid
and folding it back over the coax (which requires you to
strip back 30% to 50% more coax to get the desired finished
length - the braid shortens when it is made wider), or
by using the braid from a larger size of coax over a
smaller one.  (My first 20m version used RG-174 coax
inside the braid from some RG-59.  Best to use the
original center conductor to pull the new coax through
the braid, as it can be a royal pain otherwise.)

How long was the coax when you tuned the antenna originally?
How stable was the SWR as you moved the antenna around,
touched the test equipment, etc.?  What coax lengths have
you tried that gave a high SWR?  How did you splice them
to the antenna?

The last 2m antenna I built used about 12' of coax, and
we saw a variation in SWR as we moved the cable around
when it was connected to the SWR meter and/or analyzer.
This is one indicator that there may be RF on the shield.
(If the SWR is either LOW or HIGH, with nothing in between,
look instead for an intermittent connection.)

I've learned from long experience that, when I add a
length of coax to an antenna and the SWR changes, one
of the first things to confirm is that the coax I added
is good.  Intermittent connections are particularly
common on the shields - the SWR will be high but the
antenna will still receive signals.
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W8JI
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Posts: 9296


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« Reply #8 on: September 25, 2009, 04:11:14 PM »

I know many people use coaxial collinears using regular coax, but that won't well at all. You can't just fold the shield back over the jacket and expect it to work correctly. You also can't cut and alternate the shield and center to make a long coaxial collinear and expect that to work.

The reason is the velocity factor INSIDE the shield has to equal the velocity factor OUTSIDE the shield or the sections are not the same electrical skirt length (differential mode) as they are common mode electrical length.

Phelps Dodge, in their early solid dielectric antennas, enclosed the OUTSIDE of the cable in very thick wax inside a hollow radome. The wax had about the same velocity factor as the solid dielectric cable did on the inside of the coax, so the system satisfied the requirement of Vp inside being the c same as Vp on the outside.

Later Phelps Dodge went to air dielectric coax inside, and used air dielectric outside the bare coax. It was protected by a hollow radome, instead of one filled with wax.

I see Ham articles all the time that mess this up. Well intentioned authors think the shield can be folded back over a vinyl jacket and made 1/4 wave long, and that through some magical ignorance of velocity factor the "stub" formed by the inside of the shield will also electrically be 1/4 wave long. That will not happen.

If there is mostly air outside the cable, you need air inside the cable. If there is solid dielectric with a Vf of .64 inside the cable, you need a very THICK solid dielectric sheath over the outside of the cable that provides roughly the same .64 Vf.

If I wanted to build a single section skirt coaxial vertical I'd use a fairly large metal tube as the skirt, with lots of air inside the tube. That would make the electrical length INSIDE the skirt nearly identical to the electrical length OUTSIDE the skirt tube.

With multiple series transposed sections making a long collinear you have to do what they did in the Station Master antennas, and use the same Vf coax as the outside of the coax has. This means a few inches thick dielectric outside the cable for slower Vp cables or an air dielectric outside for air dielectric cables.


Tom
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WB2WIK
Member

Posts: 20611




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« Reply #9 on: September 25, 2009, 05:38:51 PM »

>RE: Coax Vertical for 2m question  Reply  
by W8JI on September 25, 2009  Mail this to a friend!  
I know many people use coaxial collinears using regular coax, but that won't well at all. You can't just fold the shield back over the jacket and expect it to work correctly. You also can't cut and alternate the shield and center to make a long coaxial collinear and expect that to work.

The reason is the velocity factor INSIDE the shield has to equal the velocity factor OUTSIDE the shield or the sections are not the same electrical skirt length (differential mode) as they are common mode electrical length.

Phelps Dodge, in their early solid dielectric antennas, enclosed the OUTSIDE of the cable in very thick wax inside a hollow radome. The wax had about the same velocity factor as the solid dielectric cable did on the inside of the coax, so the system satisfied the requirement of Vp inside being the c same as Vp on the outside.

Later Phelps Dodge went to air dielectric coax inside, and used air dielectric outside the bare coax. It was protected by a hollow radome, instead of one filled with wax.<

::This was done years before the company was Phelps-Dodge.  Originally, it was CPC (Communications Products Corporation) and I worked there in my youth.  The solid dielectric coax version was the Stationmaster and the air dielectric version was the Super Stationmaster, which had greater bandwidth (but no additional gain).

I recall Antenna Specialists in the 60s (maybe into the 70s) had a popular line of mobile coaxial verticals where the upper radiator was an SS whip and the skirt was about 1" diameter tubing; they worked very well but had a power limitation due to the potential at the bottom of the skirt.  We used to modify these to add a Teflon grommet between the coax exiting the skirt (sleeve) and the sleeve itself, and then it could handle a couple of hundred watts.

73

Steve WB2WIK/6
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WB6BYU
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Posts: 13335




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« Reply #10 on: September 26, 2009, 07:11:09 AM »

W8JI wrote:

>  The reason is the velocity factor INSIDE the shield has to equal the velocity factor OUTSIDE the shield...

Tom -

I presume this applies to coaxial colinear antennas where
a coaxial section is used both as a transmission line and
a radiator, rather than to a simple dipole.  (Illustrating
again that there are many different designs that can be
called a "coaxial antenna.")

But it is a good point, and demonstrates one of the reasons
for poor decoupling on a coaxial dipole when the shield
is simply folded back over the outside of the coax:  the
length of shield required for quarter wave resonance
for the dipole is different than the shield length
required for a quarter wave decoupling sleeve due to
the dielectric effect of the outer insulation of the
coax (which affects the sleeve but not the dipole.)
One solution is to add a second sleeve (or radials)
below the first, as is often done on commercial antennas.
This does ruin the simplicity for home construction,
however.

Probably the best approach for a dipole is to use a
large enough conductor for the bottom section so that
most of the dielectric is air, which improves the
decoupling effect as well as reducing losses in the
vinyl insulation on the coax and provides a higher
voltage rating across the open end of the sleeve.
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