[TowerTalk] YCCC 9 circle preamp failure test

[jimlux]

On 9/24/19 3:55 PM, Wes wrote:
> On 9/24/2019 1:36 PM, jimlux wrote:
>> On 9/24/19 1:14 PM, Wes wrote:
>>> Why all of the complications?  The preamps are unity gain. 
>>
>> Aha.. I was hunting around for a circuit description. Yeah, it looks 
>> there's a noninverting buffer followed by a 1:1 transformer.
>>
>> So it's more of an impedance buffer.  What Z does it present to the 
>> element?
>>
>> (I do a lot of work with HF receivers that use short (2-3 meter) 
>> antennas feeding high Z buffers, so they're more "E-field" probes)
> 
>   Can't be more than 50K ohm (the parallel combination of the two bias 
> resistors).  I don't know what the 5pF cap is doing, maybe suppressing 
> something or the other, otherwise, it's contraindicated.   IMHO, of course.
> 
> Wes  N7WS
> 


So on the low bands the reactance of that small cap is going to dominate 
the voltage divider ratio of Zin/(Zin+Zant)
lambda	f	Xcap
40	7	4.8k
80	3.5	9.7k
160	1.8	19k

I also saw a version of the schematic with 50pF


A 20 foot vertical has a approx Zfeed of
40	7	10.4-160j
80	3.5	2.3-380j
160	1.8	0.6-660j

So the voltage divider ratio is probably above 90% for all these cases.

Considering it as unity gain is "close enough" and you've basically got 
a 5-6 meter long field probe

---
Let's look at Tx power and the field at the Rx site

THe distance is 100 ft, let's call that 30 meters
Let's say he's radiating 1kW, so 1kW spread over a hemisphere that is 30 
meters in radius is: 0.18 W/square meter, or about 83 microvolts/meter

83 microvolts/meter * 6 meter probe length = about 500 microvolts -> I 
don't think you're going to burn out anything with that.

Even if he has a 10dBi Tx antenna beaming at the receive site, that's 
going to bump it to about 1.5 mV


So if input capacitors are failing, it's not because of RF voltage in 
low bands.

But let's consider an interesting case.. the elements are 20 feet long.. 
rounding to 6 meters, they'd be a half wavelength long at 12 meters 
frequency - In reality, they'll resonate somewhere else, but in that 
ballpark..It might be 10 meters, or CB, or something else..  If the 
elements are a bit longer, or there's some capacitive loading, the 
resonant frequency might drop a bit - maybe it resonates on 15m at 21 MHz?

Now you've got a resonant dipole, with high voltages at the endpoints. 
If your Tx happens to excite the resonance, then you might get some 
substantial voltages.. You've got a high Z load.


Shorting the element to ground when not in use would fix this.
Some sort of high pass shunt to ground (a capacitor) might fix this - 
maybe that's what the original circuit's 50 pF was for, although it was 
after the series blocking cap. But maybe that combination is enough to 
clamp the element voltage at the feed point.







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