[TowerTalk] Is There a Good Balun...

[Jim Lux]

On 11/26/14, 9:48 AM, Wilson wrote:
> I have asked my resident experts (U know who u r.) and added my own
> unschooled opinion.
>
> For the three band range (20-10) it’s really trivial. Going on down
> to 30 and 40 is only slightly more tricky. The ARRL Antenna Book
> deals with this quite concisely and gives winding data for the “best”
> chokes for each situation. Looking a little deeper, they say that if
> the coax goes down the mast, so it doesn’t pick up and reradiate a
> lot of energy (which compromises the pattern), the balun isn’t really
> needed.  So maybe someone will swing some patterns with and without
> the choke, for us?
>




> My own take is that after hearing a lot of talk, much of it BS, and
> being exposed to a lot of colorful ads, we have developed a “need”
> that far overshadows reality.

I think there's also an element of tradition and perhaps misapplication 
of a component that is necessary in one situation and not in another, 
and which doesn't necessarily cause any performance difference.

Say you try a choke/balun/transformer/matching network, you climb the 
tower, put one in, climb down, and it works about the same as it did 
before. Perhaps because underneath it all, you didn't actually need it. 
  You're probably not going to climb back up and take it off: you just 
leave it..

Then, someone else copies your setup ( W6RMK and W4EF have the parts 
installed, and they just swept all bands in the last contest with their 
novel SO2R station at W6VIO using a 10 watt transmitter, a diode 
detector and an earphone, using strips from a beer can as the key, so 
that must be the hot ticket, I'm going to copy it here at W1AW)

Doesn't hurt, nor does it help, and so the legend is established.  All 
the cool kids have a balun, so I need one too.

---
here's the other reason why I think folks want a Balun..

For the impedance transformation:  There's a lot of antenna designs that 
work reasonably well ("work" as in meeting some installation constraints 
and producing an acceptable pattern, etc.) but do not have a 50 ohm 
feedpoint impedance.  For instance, a folded dipole has a 200-600 ohm 
feedpoint impedance (depending on the spacing of the folded wires and 
how many of them there are).

A vertical against a really good ground plane has a 30 ohm feedpoint 
impedance.

Many Yagi designs have VERY low feedpoint Z, if there's no matching 
network at the feed.

So a designer might say, I'll take that 10-15 ohm feedpoint impedance, 
put a 2:1 turns ratio transformer and get it nicely up into the 50 ohm 
range, without having to fool with gammas, deltas, tuning bars, etc. 
And, as a side effect, the transformer will also deal with any common 
mode issues because it will act as a choke.

Or, an offcenter fed dipole might have a Z of several hundred ohms at 
the feedpoint: so a 3:1 or 4:1 turns ratio (9:1, 16:1 Z ratio) 
transformer makes for a better match to a 50 ohm feedline.

This is especially so since a lot of hams have no other way to evaluate 
the antenna than looking at the VSWR bandwidth. So manufacturers have an 
incentive to try and design something that presents a good match over 
the entire band.  In the worst case, this leads to cynical devices like 
the MaxCom antenna matching unit with a resistor in it.



>
> And if the coax choke works, why on earth would one want to spend
> $50-$100, which will buy a LOT of beer, on $10 worth of stuff crammed
> into a little box AND four more connectors to get wet? I think
> there’s a real possibility that the losses in the fancy balun, not to
> mention poor ones, may be more of a problem than the pattern
> distortion caused by not having one.

$50-100 is a perfectly reasonable retail price for $10 (or even $5) 
worth of parts (labor, storage, marketing, etc.)..

However, you do raise a valid point of increasing failure probability 
with components in a hard to reach location and potentially higher loss.