[TowerTalk] Fwd: Tower and antenna decisions

[Roger (K8RI) on TT]

On 10/28/2013 9:26 AM, Jim Lux wrote:
> On 10/28/13 12:31 AM, Michael Tope wrote:
>> On 10/27/2013 11:01 AM, Jim Lux wrote:
>>>
>>> Whether it will take the SWR to 1:1 is a function of the step size in
>>> the tuner design (if it's a switched L and C) and the control 
>>> algorithm.
>>>
>>> Most autotuners stop when the SWR is below, say, 1.3:1 or 1.2:1.
>>>
>>> What you might want is a way to manually configure the L and C, and
>>> then store that for the frequency (most tuners can do this now.. the
>>> AT200PC can, for sure), or have a computer that knows what frequency
>>> you're tuned to, and then set it up.
>>>
>>> (this is what I was doing with my active phased array.. I used AT200s
>>> as essentially computer controlled LC networks)
>>>

Thanks for the ideas, Jim and Michael.
>>
>> If you are shooting for really close to 1:1 VSWR to keep an super
>> finicky SSPA happy, you probably want something with continuously
>> variable matching components in it. RF Concepts (aka Alpha Power) is
>> working on one to handle a wide impedance range:
>>
>> http://www.rfconcepts.com/PRODUCTS/New-Products/Alpha4040

I've had one on order since they were announced, but they keep promising 
and saying there is progress, but to me it's a "when, or IF" proposition 
and the tuning is still done in the shack. which with a relatively 
narrow "natural" usable frequency range and a highly imbalanced antenna 
like the fan dipole with one end at 90 feet and the other at ten, there 
will be a lot of RF on the shield. Enough to require two 5000 ohm 
current chokes.

Regardless of how close to resonance I operate that imbalance is going 
to exist.  For that, I think the stepped inductance to move the useful 
resonant frequency across the band while using the chokes/current baluns 
to get rid of the RF on the shield and a tuner in the shack to match the 
impedance is the way to go.  Although they don't show much gain or 
directivity the slopers have worked well for me when chasing DX.

>>
>> I think they are still struggling with making it work reliably over what
>> amounts to a fairly ambitious impedance range, but I am sure it as it
>> stands now it would pull in the VSWR of a Tennadyne LPY from 2:1 down to
>> 1:1 without breaking a sweat. While it's not exactly the same, the Alpha
>> 87A and 9500 as well as the Acom 2000 already do this sort of thing -
>> that is - automatically bring in a <3:1 VSWR (<2:1 for the 87A) into
>> something close enough to make the finicky ceramic tubes with delicate
>> grids happy. The auto tuning of the stepper-motor driven variable caps
>> on these products is very fast.

Unfortunately I'll be looking at a low impedance, unbalanced load which 
takes several steps to make amplifier friendly.  With out "both of those 
chokes" there is enough RF back into the shack to cause problems.  
Without moving resonance, IIRC the SWR can reach some where around 8:1 
at the extremes. Even with resonance near the center of the band the SWR 
still exceeds 3:1 by a wide margin.
The half sloper is actually easier to match as I treat it as an end fed 
wire.


>> In this case of more forgiving PAs that don't need super low VSWR, the
>> discrete tuning step approach is as you suggest perfectly fine.
>>
>
>
> That's what I was thinking..
> I think you could probably adjust the L and C under computer control 
> with stepper motors as fast as you can spin the VFO dial. They do this 
> sort of thing on big RF plasma systems (industrial processing) to keep 
> the match optimized.

I'm from the days where we did little matching<:-)) (60s and 70s) I quit 
and went to college in 87.

>
>  Big band switches might take a bit longer if you have to make big L 
> steps.  OTOH, for a "purpose designed" tuner, you could use relays to 
> switch big L in steps, and have a small variable L.
>
> Also, it's really more a matter of how small the LSB is on the relay 
> approach.  0.001 microhenry steps (0.37 ohms at 30 MHz)  are doable. 
> It's just a cost thing: how many relays do you want to buy.  Narrow 
> band is easier (you probably don't need 0.001 uH steps for top band)
>
> The challenge is in calibrating. I've measured 5 AT200PC tuners and
> a) they are not the same as each other
> b) the steps are not linearly combined.  That is, if you have a 1,2,4 
> sequence, turning on the 2 does not always add the same inductance or 
> capacitance.  It's, I suspect, coupling between the components, and 
> interaction with the wiring.

That was one nice thing about working in industry and lab equipment.  
Whether 1 or a 1000 you could count on them being the same.

>
> This isn't a problem in the "autotuner" because the search algorithm 
> can always find a minimum, and as long as the steps are always getting 
> smaller (i.e. it's monotonic) then it works.
>
> As a result, though, you can't set a particular L and C in an a-priori 
> sense.  For the LPDA/Yagi matching scenario, you'd have to make a cal 
> run, and store the actual values you need for each frequency.

Which "generally" works well and is a good approach unless the antenna 
is sensitive to the environment and weather.  It can be a PITA on 160 
and 75, but there is no guarantee.  The specific antenna might be quite 
stable.

>
> This is really no different than the SteppIR thing, if you tweak the 
> factory default settings.

Other than the StepperIR has more moving parts.  I've stayed away from 
them for that reason, but a big crank-up, tilt over tower and the beam 
mounted on a tilt plate should make maintenance much easier.

>
> For the phased array scheme, it's a WHOLE lot more complex because of 
> the mutual impedances between elements (that is, changing the match at 
> the feedpoint of element A changes the Z at feedpoints of elements B, 
> C, and D).  It starts to get very complex, because you now need to 
> measure the element current mag and phase in real time. (A pain in the 
> rear 20 years ago, not so much today, though)

True, but you can easily end up with the element matching points chasing 
each other.  IE a change at element A requires a change at element B, 
but that requires a change at element A.  If the Algorithm does this in 
a set order and the size of the steps is based on the magnitude of the 
excursion (steadily reducing the magnitude of the steps) should prevent 
that.
>
> This is why I'm thinking that synthesizing the reactive component with 
> a suitable PA, rather than using switchable passive components for 
> "power factor correction", is a nice idea. (it also lends itself to 
> very rapid frequency changes)

I think this is a good approach except in cases like the low impedance, 
sloping dipole with its imbalance.  Low frequency, low impedance, 
imbalanced antennas are best handled at the antenna, or the solution is 
split between the antenna and shack.

73,

Roger (K8RI)

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