[TowerTalk] Fwd: Tower and antenna decisions

[Roger (K8RI) on TT]

On 10/27/2013 2:01 PM, Jim Lux wrote:
> On 10/27/13 10:25 AM, Roger (K8RI) on TT wrote:
>> On 10/27/2013 10:10 AM, Jim Lux wrote:
>>> On 10/26/13 8:15 PM, Hans Hammarquist wrote:
>>>> All these "modern", solid state, PA have the same problem, they are
>>>> "protected" and the "protection" rolls back the power as soon as they
>>>> detect reflected power. Little depending on make and design they roll
>>>> back more or less. That's why the manufacturer offer built in tuners.
>>>> The "old days" with a pi-filter on the output could be tuned to most
>>>> anything below SWR of 1:3 or even more, and they didn't have (needed
>>>> maybe) the "protection". As long as you didn't kill the final tubes
>>>> by overheating them, you were OK. Do we like to have the "old" tube
>>>> final back? Maybe.
>>>>
>>>
>>> I would rather have "smart antennas" with the finals *at the antenna*
>>> and the matching done there.
>>
>> This gets back to what I want to do...sorta.
>> Particularly on 160 you don't have a lot of room to make frequency
>> excursions and that is to put the tuner "at the antenna", but that comes
>> with a location that is hazardous to the tuner's health. Another is just
>> how good are the remote autotuners?  Will they take the SWR right down
>> to 1:1 which is important for SS amps, not because of power, but because
>> of deteriorating signal quality.
>>
>
> 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.

Understand.

>
> Most autotuners stop when the SWR is below, say, 1.3:1 or 1.2:1.
>
Which is not good enough for me. I tune the amp into a non reactive 
dummy load with an SWR of 1:1.00XX and expect it to see the same when 
switched to the antenna.

> 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.
The problem there is lack of ambition and programming with only one 
hand. Before my medical inconvenience I could type over 70 WPM while one 
handed typing is more than a little annoying.

I have a BS in CS with work toward a masters before a good job came 
along and ended my quest for a Masters, so I enjoy working on these 
types of problems. It's just the one handed typing really slows me 
down.  Write the Algorithm and then write a A Nassi-Shneiderman 
diagram/flow chart.  Makes multilevel programs much simpler.

My solid state rigs all auto tune to very close to a 1:1, so I expect 
the same from an external tuner.

>
> (this is what I was doing with my active phased array.. I used AT200s 
> as essentially computer controlled LC networks)
>
>
>
>> With a remote tuner, I want to match the antenna impedance, not just
>> move the resonant point.  Yes, if I move the resonant point to cover the
>> entire band it does make life easier and I could take care of the rest
>> in the shack, but again I'd prefer to do this at the antenna so in most
>> cases I only need a small L network even for 160 if it's close to
>> resonance.
>>
>> With the half sloper other than the difficult maintenance problem this
>> becomes rather easy although on 160 that makes for a lot of resonant
>> points.
>>
>> putting the matching network at the antenna for a center fed, half wave,
>> sloping dipole is not practical although a single band tuner at the
>> tower using open wire line might.
>
> I've looked at this strategy for portable operation with a 40 foot 
> extendable carbon mast.  Basically, put the tuner at the base, run two 
> wires up the mast to the dipole.  Sure, that feedline has some weird 
> impedance and varies, but because the Z is "high", the currents are 
> fairly low, and it's short, in any case. Modeling shows that it should 
> work pretty well (e.g. no significant loss compared to putting the 
> tuner at the feedpoint).  The tuner takes care of whatever weird 
> impedance is presented.
>
>
>
>
>
>  Ice storms are common here spring and
>> fall, although there are far fewer in the fall but the make open wire
>> problematic and to me, reliability/durability is important because I
>> have to impose on others to get things fixed.
>
>
> The SGC tuners are pretty rugged devices in a pretty rugged enclosure. 
> Sure, it *is* moving parts (relays), but I think that if we put our 
> minds to it, we can design this kind of thing and have it be rock 
> solid reliable.  No, some cheesy breadboard in a rubbermaid box isn't 
> going to hack it.

And ruggedness adds weight, but I agree, this is no place for the bread 
board approach, except during the design phase.

>
>
>
>
>>
>>>
>>> Sure, it's more complex than the historic Transmitter in
>>> Shack/Feedline/Fixed Antenna, but life moves on.
>>>
>>> For instance, I sketched out an interesting design for a form of Yagi
>>> with all driven elements, using an array of magnetic loops, rather
>>> than the traditional horizontal elements. The matching from low Z
>>> semiconductors to the low Z of the magnetic loop is actually kind of
>>> what you want.   And you're doing spatial combining, so with 5
>>> elements, each driven with a 200W module, you don't have the losses in
>>> the power combiner you see in a "single output" SSPA.
>>>
>>>
>>> Combine this with things like polar modulation, and you can get some
>>> very interesting designs. It's almost like having the entire rig at
>>> the top of the tower, and all you need is power and an ethernet link,
>>> which could be wireless.
>>>
>>> Sure, its nothing like ham radio in the past, but that's what ham
>>> radio is all about: try new things.
>>
>> I like the idea and to me it's far less different than a remote regular
>> station, controlled over the internet. You're just combining the rig
>> with the antenna. There might be issues with lightening, maintenance,
>> and cost though.  Could it be made to match the big mono band Yagi for
>> performance?
>
>
> Ah.. there's the rub.  The single biggest factor in performance is 
> height above the ground.  The second biggest factor is number of 
> elements (which sets the F/B ratio) and length of boom (which sets 
> gain).  the element length isn't a big driver of gain.
> An infinitesimally small dipole has a directivity of 1.6dBi, and a 
> full size dipole has a directivity of 2.15 dBi.  So that 0.5 dB is 
> something you'd have to make up some other way (more elements, longer 
> boom); but fundamentally, there's no inherent advantage in full length 
> elements. In a Yagi-Uda, you also want full size elements so that you 
> can couple power among the elements easily, with the right amplitude 
> and phases, without having tight mechanical tolerances.
>
> The other factor is efficiency.  Physically small radiators often have 
> high losses, and the way the amateur regulations are written, that 
> presents a problem. If there was a "radiated power" limit instead of a 
> "transmitted power" limit, then it would be easier.
>

At least we no longer have amplifier efficiency paramount, but can move 
toward class A for the best signal, although if you want an amp with a 
clean signal you almost have to build it and there are some good tubes 
out there.

> If you're not concerned about wall plug efficiency, then you can 
> *radiate* just as much power from a small loop as a big dipole. All 
> either does is match free space to your feedline.
>
>
>
> The challenge is in getting what's called superdirective gain (e.g. 
> gain more than N where N is the number of elements (5 dB for 3 
> elements + 2.15 dB for the dipole = 7.15 dBi)..  To get superdirective 
> gain, you generally need to have mutual coupling among the elements, 
> which is how Yagi-Uda's "work".  An all driven array can do this, but 
> you have the prospect of elements with negative power (something 
> familiar to folks building directional arrays in the broadcast business).
>
> Most amplifier designs for RF don't have a way to feed power back to 
> the DC bus, for instance; although this is now standard practice in 
> motor drive systems (so called 4 quadrant drives). So your negative 
> power element would need to dump power into a load.
>
> But hey, with these new fancy LDMOS devices with huge SOAs and very 
> fast transition times, we can start to look at RFPAs as more like 
> power switching systems, rather than narrow band tuned amplifiers.
>
73,

Roger (K8RI)

>
>>
>> It's a radical design, but used much the same way as ham rigs have since
>> day one.
>>
>>
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