[TowerTalk] elevated short vertical dipole or quarterwave monopole?

Tom Rauch w8ji at contesting.com
Thu Dec 2 21:17:38 EST 2004

> Are you saying that it is not possible to decouple with a
coaxial sleeve
> such as is commonly used on VHF or UHF collinear antennas

No. It doesn't decouple well but it decouples.

It decouples because the coax inside the sleeve is 1/4 wl
long electrically.

This creates a high impedance effectively in series with the
shield for common mode.

That impedance depends on the loss between the outside of
the coax and the inside of the sleeve, the surge impedance
of the coaxial line formed by the shield and the sleeve, and
the electrical length of the sleeve over the cable.

If the cable is not electrically 1/4 wl long to the end of
the sleeve, the shield of the coax has greatly reduced
decoupling impedance.

If you run the cable down the INSIDE of a short dipole, the
OUTSIDE of the sleeve (the bottom half of the dipole) may be
resonant by loading and electrically 1/4 wl long..... but
the INSIDE of the sleeve is only whatever length the cable
and the inside of the sleeve form. The loading on the
outside has no effect.

There are VERY special things that muct be done to make
sleeve decoupling work, and peoploe often just don't grasp
the concept at all. For exaple if you fold the shield of a
cable back over the PVC jacket without a significant air gap
between the cable and the inside of the folded shield the Vf
of the jacket makes the inside of the sleeve look longer
than it is. The high loss also limits impedance, and the
thing is "crapola" for decoupling.

In contrast the Isopole vertical used a large air gap,
flared the open end to increase surge impedance, and did
things right. Despite doing that they still found a second
sleeve was necessary or useful.

On one hand we have people not knowing what the sleeve does
and how important it is that the electrical length INSIDE
the sleeve be 1/4 wl and the loss low and impedance high
folding back shields thinking they are decoupling something.
On the other hand (Isopole) we had people who understood the
problem and did things right.

It was used
> for many years quite successfully as a quarter wave VHF
sleeve decoupled
> mobile antenna by Bell telephone. Or are you referring
only to a
> vertical dipole sleeve antenna that is mounted close to
the ground?

Actually the single sleeve antenna you refer to has
considerable spacing between the internal "feedline" and the
sleeve. The sleeve electrically is the same length inside as

Despite that, it still had considerable common mode current.
You can actually make collinear antennas by stacking
sleeves, the leakage is so high.

Check out:

and you can see a skirt collinear at


When you load an antenna, you don't even have a 1/4 wl
decoupling stub because the inside of the bottom of the
antenna is not 1/4 wl long!

I measured several loaded antennas and it made no difference
if the feedline ran down the inside or down the outside of
the bottom tubing. Same reason the Gap antenna has high
common mode on the feeder.

73 Tom

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