Topband: 160 meter elevated vertical

[Guy Olinger K2AV]

Hi, Bill,

I would add that there is nothing good that can come of having coax pull
away from a vertical without a good common mode choking device of some
sort.  You may get away without it, but if not you have lost.  Lost dB,
picked up noise, changed pattern to favor noise sources.

If you have raised radials, you do NOT want dirt functioning as any part of
your counterpoise.  The helpful aspect of radials is creating fields in the
ground that are counter to the fields created by the vertical radiator.
 Uniform all around radials create a uniform all around field in the ground
which is opposite to the uniform all around field in the ground from the
radiator.  Their cancelling out is worth 2 dB with "good" radials.    If
all the shield current has to go to the radials and all the center
conductor current has to go to the vertical, then the sum of currents in
the radials is equal to the current in the vertical.

As soon as you let current flow on the outside of the shield away from the
feedpoint, you are taking it away from the radials, the degree of field
cancellation in the ground lessens and you are throwing away RF.

And not all ground is equal.  Ground rods can work very poorly for RF.

Many situations will require more than one choke to be effective, or
strategically placed choke and ground.  But a vertical with 1/4 wave raised
radials and a common mode choke at the coax feed is a quarter wave from
ends of wire and a near-perfect place to do the job with only one choke.
 And you don't need a hail-mary super choke for the job.

Just DO the common mode choke on any vertical.  Don't tempt Murphy.

73, Guy

On Fri, Oct 19, 2012 at 9:38 PM, Tom W8JI <w8ji at w8ji.com> wrote:

> I'll be putting up a T-top elevated vertical for 160 in the next couple of
>> days. The baser will be around 60 ft above ground and the Tee (about 40
>> feet  long will be at about 170-180 ft...depending on the caternary sag.
>> There
>> will be  4 radials at 90 degrees to each other.
>>
>> Question: Any type of balun necessary with this? Or is it good to go. I'm
>> using RG58 to feed it to keep the weight low on the caternary to avoid say
>> but  could wind something on a four inch PVC.
>>
>
> The goal of ANY common-mode choke is simply to make the path to ground
> along the outside of a cable look like a poor path compared to a ground or
> counterpoise at each end.  The common mode impedance, exciting source, and
> location of the choke dictate what is required.
>
> All verticals with sparse ground systems need common mode isolation,
> unless something causes the feeder to look like a very high impedance
> compared to the path out to the radial to earth voltage. Your antenna base
> is around 1/8th wave high, and I assume suspended in the clear vertically.
>
> If you simply ground the coax shield to a reasonable earth ground at the
> earth surface, you will already have a pretty good CM choke for your
> system. The feeder itself will act like a choke. If you wanted to add
> anything, a little inductance anywhere along that cable shield (by coiling
> the cable) would make things better.
>
> 73 Tom
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