[TowerTalk] To get a truly balanced antenna feed

[Roger (K8RI) on TT]

That's what I truly like about an AIM.  It gives you all the important 
figures about your antenna and at the frequencies you operate. Z, R, 
SWR, Return loss, loss, impedance bumps, mismatches and their locations 
and more. Each parameter is graphed for the entire band, or in the case 
of a multi band antenna, the resonance points for each band. Properly 
calibrated, it can show any problem in the system and the distance to it.

With the aim, although small and inconsequential, you can even see the 
effects of nearby objects, even trees moving in the wind. Some, like 
another antenna may not be inconsequential, at distances farther than 
you'd expect.  You simply save the curves for the one antenna(s) before 
and after, then overlay the displays. Any change really stands out.

This is particularly helpful with an antenna such as the AV-640 which is 
quite sensitive to nearby objects and very sensitive to other antennas, 
even horizontal ones.

The effect of frequency on imbalance becomes quite apparent with 
multiple bands on the same feedline.  The difference between 80 and 40 
meters is, for lack of a better term, "astounding", particularly when 
the antennas are sloping, center fed, dipoles.  On 40 meters, a simple 
choke built according to the formulas in K9YC's tutorial is all that is 
needed, but on 80 it takes two chokes in series to keep RF out of the 
shack.   There, I have doubts that chokes with more turns and only two 
cores could handle the dissipation.

73

Roger (K8RI)


On 6/22/2016 Wednesday 9:24 AM, jimlux wrote:
> On 6/21/16 11:21 PM, Jim Brown wrote:
>> On Tue,6/21/2016 9:31 PM, Roger (K8RI) on TT wrote:
>>> Didn't you once say "there is no such thing as a balanced antenna"?
>>
>> Yep, but I was speaking more in terms of perfection -- i.e., that in the
>> real world, most practical antennas are unbalanced by their
>> surroundings, even if we attempt to make them a balanced antenna and
>> feed them with 2-wire line and a so-called balance tuner. It is that
>> imbalance that causes common mode current. If the imbalance is
>> relatively small, and if the antenna is resonant and fed with coax, we
>> can choke it effectively.
>>
>
> Which brings up an interesting thing..
> one reason that folks want to use a balanced transmission line is that 
> it's low loss (mostly because it's high Z, so the current is low) 
> because the antenna is not well matched.  You can use a tuner in the 
> shack (or the plate output tank on a tube rig) to do the matching 
> without paying a huge penalty in transmission line loss.
>
> On the other hand, it's hard to make an effective choke for open wire 
> line, compared to the simplicity of just putting a bunch of turns of 
> coax through a suitable core.
>
> I'd also contend that a lot of the claims of "excessive loss" or "low 
> loss" are based on an over simplified model of the system: someone 
> looks up the "loss per 100 ft" number and uses that. But that number 
> usually assumes an integral number of wavelengths.  In a mismatched 
> line, the current and voltage fluctuates along the line, so the loss 
> per unit length varies (at HF, the dominant loss is IR loss).
>
>
>
>
>
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-- 

73

Roger (K8RI)


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