Hi Stefan,
I am not measuring antenna current.
I am using the M0BMU ScopeMatch immediately following the LPF. The
version I built has a transformer for monitoring current and
capacitive divider for voltage. Bot current and voltage are
monitored at all times using a two channel scope.
What I see during Tx is that current (on the feedline) continually
increases while voltage remains constant or maybe decreases a very
small amount. The product of voltage * current (power) increases.
At the same time, voltage and current meters on the power supply
show the PA drawing more current and consuming more power as the Tx
cycle continues. This is what I would expect for class D or E PA if
the load impedance is decreasing.
All of this happens with both PAs. But it doesn't happen if I
replace the antenna by a dummy load.
Of course I do need to measure *antenna* current to be sure it is
increasing as I expect. My old antenna current meter has died. I
bought another two weeks ago from eBay but it arrived dead as is
often the case with those old meters. I am going to buy another one
as soon as today! And hope this one will be OK. I will let you know
when I have been able to measure antenna current again. I will be
surprised if I do not find it slowly increasing during Tx periods.
73,
Paul
On 12/3/18 8:03 AM, DK7FC wrote:
> Hi Paul,
>
> How do you measure your antenna current? I know, the scope. But do you
> measure across a small shunt resistor or do you use a current xfmr or
> something else?
>
> 73, Stefan
>
> Am 03.12.2018 12:49, schrieb N1BUG:
>> Hi LF,
>>
>> During past days I did some more investigating about that slow
>> antenna R change during Tx. This is making me a little crazy. I
>> don't like RF mysteries!
>>
>> During a 2 minute Tx at 200W, antenna system R reduces about 20%
>> from beginning to end of Tx.
>>
>> During a 2 minute Tx at 100W, antenna system R reduces about 20%
>> from beginning to end of Tx.
>>
>> During a 2 minute Tx at 50W, antenna system R *increases* about 3%
>> from beginning to end of Tx!
>>
>> There is almost no shift in X.
>>
>> I went over everything I could get to (matching xfmr, loading coil,
>> connections between them, etc.) using a IR heat measuring gun. I did
>> not find anything warming up during a long Tx period. It's all cold.
>>
>> I see a similar change happening on MF where I use a completely
>> separate xfmr and loading coil.
>>
>> This may be a clue, but I don't know what it means. I have other
>> antennas around. All those cables come into the basement where there
>> is a disconnect point. All can be disconnected from the short cables
>> which run up into the radio rooms. If I disconnect all other cables
>> at that point, then the LF antenna R change during Tx is about 10%,
>> half what it is usually. The R at the start of a Tx period is the
>> same as always, but it does not decrease as much during the Tx period.
>>
>> I tried connecting the coax shield to the LF antenna ground. That
>> did not make any difference.
>>
>> Just trying to think of possible explanations...
>>
>> Broken or bad connection somewhere up on the top hat of the antenna?
>> Seems something should fall down if so, but it hasn't yet. :)
>>
>> Bad connection in the ground system? All above ground connections
>> are secure. Below ground connections are not accessible in winter
>> but they are heavy solid copper conductors, mostly exothermic welded
>> connections. The one exception is the heavy solid wire which runs
>> from the xfmr secondary down to the underground wire / center ground
>> rod. It is clamped, not welded. But I used three clamps, very tight
>> and with anti-oxidant paste. This connection is just one year old.
>>
>> Ground return currents through some other path which is not stable?
>> I am sure I make RF currents in the power lines, etc. but...???
>>
>> Does anyone see a clue here?
>>
>> 73,
>> Paul
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