[TowerTalk] Coils vs Linear loading on 80M yagi

[Tom Rauch]

Hi Charlie,

> There has been a long thread about coils Vs linear loading on lower
> frequency yagis.  I think it is too bad that the impracticalities of
> having an A/B switch for the larger yagis makes actual testing a bit of a
> pain and rules out all but the most exotic stations from even thinking
> about this. However there are several good hints that one has when a yagi
> works or doesn't work and so I would like to relate an anecdotal story
> here because I believe it has some merit.

Measuring an actual antenna which always has many errors. We 
often like to pretend the results are absolute, but they are not. 
There are almost always a few dB of error or scatter in HF antenna 
measurements, even in a controlled environment.  

Fortunately, we can measure the loss of a system that is part of an 
antenna quite easily and accurately. We can also calculate the 
loss.

One thing we have to consider is that linear loading systems 
radiate. As such, the move the effective point of loading. 

1.) If you attach a linear loading system halfway out on an element 
and fold it OUT towards the ends, the effective point of the load is 
moved out. That's a good thing.

2.) If you fold that linear loading back in towards the boom, it 
effectively moves the loading back in and that's generally a bad 
thing.

So if we keep all things equal, and load with an inductor at the 
same **effective** point on the element, we only need to consider 
ESR. The reactance required is the same in either case, if the 
point of loading is the same. With reactance being the same, ESR 
relates directly to Q.

A typical 400 ohm reactance stub of #14 copper wire has a Q 
between 20 and 50 on 40 meters.

A typical coil with the same reactance and wire size has a Q of 
100-250.

A well designed coil has an upper limit Q of about 700, no matter 
what size wire you use. With wire as small as number 8, you could 
get a Q of 500 or so in a good design.

The only saving grace for linear loading is it lets you effectively 
move the load out on the element if you implement it correctly, 
without adding an insulator out on the element. That would occur, 
for example, if you insulated the element near the boom and then 
had the linear loading wire extend out on the element and then 
back to the boom. You'd need a higher Q coil to equal the linear 
loading, but that's easy to do.

If we did it the other way, and insulated the element out at a 
distance and folded the loading back towards the boom, the 
loading point would be moved in. A coil with the same exact Q (and 
that would be a pathetic coil) would actually be better.   

Looking at mobile shootout is a good lesson in antenna loading. 

You'll see the antennas with a lot of end-capacitance are 
consistently more effective no matter where the coil is placed, and 
attempts at linear loading fall short of base loaded systems.

The same concept, applied to yagis, would be a good one. With a 
large but light-weight capacitance "hat" at the element ends 
(perhaps stainless steel whips), you could put the coil *anywhere* 
on the antenna element with very little change in performance. 
You'd have maximum bandwidth and minimum loss, and the coil at 
a convenient point to adjust and mechanically mount.

Cushcraft sorta does that with a 40 meter antenna (although not to 
the extreme that I would), but I'm amazed that I don't see antenna 
manufacturers offer an 80 meter design like that. Efficiency would 
become higher along with a wider bandwidth, and the antenna 
would be more performance-stable if the element flapped around a 
bit.

I guess it must be a marketing thing. Everyone is probably 
brainwashed into thinking linear loading is "lossless" or "lowloss".


73, Tom W8JI
w8ji@contesting.com

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