[TowerTalk] elevated short vertical dipole or
jimlux at earthlink.net
Thu Dec 2 22:18:38 EST 2004
> 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.
Other approaches are careful design of the transmission line inside the
bottom half of the dipole. The usual run of the mill UHF/VHF antenna isn't
loaded, and so, getting the coax to be 1/4 wave long isn't so difficult (in
fact, getting it to be as short as 1/4 wave is tricky... air dielectric
helps). However, if the dipole is short (electrically), then you need to
have some dielectric in there. Say your dipole is 1/2 length (i.e. 1/8
freespace on a side).. you'd need a dielectric with an epsilon of 4 to make
this work (since the velocity goes as the square root of epsilon). It's a
challenge finding low loss, high epsilon materials. Even tougher to make a
sleeve decoupler work over a wide band, although I should think that one
might be able to find some physical configuration where everything sort of
cancels out, at least over 10% BW.
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