Thanks Jay and Laurence for weighing in on this.
Like many amateur radio operators I am not likely to have
opportunity to take field strength measurements, though I wish I could.
It seems to me it must be trees rather than the earth itself that
accounts for most of the changes I see. The sap retreating makes
sense, though I am somewhat surprised by how quickly the change
happens with temperature. The very large scale change around the
freezing mark and just below also strikes me as being somewhat curious.
I was hoping there might be some evidence collected with regard to
correlation between estimated / calculated EIRP and actual field
strength measurements. I presume it can never be more than the
calculated level unless the antenna is directional... but, perhaps
measurements might reveal lower EIRP than the calculation when
dealing with certain environmental factors such as trees. I'm not at
all sure. Maybe all of the losses due to trees show up in resistance
measurements? I may never know, but it is an interesting subject.
In any case I am running the full one watt EIRP as calculated based
on measured system parameters. Given my results to date I don't
think I can be too many dB below the calculated level! :) I have
plenty of trees, mainly pine, oak, maple and birch but with a few
others here and there.
I had to shut down last night due to our first ice storm. I couldn't
keep up with rapid antenna resonance changes. I should be back to
transmitting tonight.
73,
Paul
On 10/27/18 10:58 AM, Laurence KL7 L wrote:
> I would absolutely agree with Jay - and your comments Paul -
> proportional and differential - the trees react very fast to
> external temps and clearly seen on losses - The White Birch
> especially, my Conifers not so much
>
> ive reduced the variability but earthmatting to close to 100ft
> radius and then tying all the extending radials out to 200ft with
> a perimeter ring wire - this seems to have stabilized wx
> variables a lot - that and a chainsaw
>
> Laurence KL7L
>
>> On Oct 27, 2018, at 3:47 AM, "[email protected]"
>> <[email protected]> wrote:
>>
>> Paul
>>
>>> I am no expert but I guess the trees are partly responsible
>>> for the high resistance and the drop with freezing
>>> temperature?
>>
>> It's been my 'hypothesis' that as the temperature cools the sap
>> heads toward the roots in stages ... not in one big rush. As
>> the temperature heads toward freezing, sap first leaves the
>> small branches and small leaders heading for thicker parts of
>> the tree. As the temperature further decreases below freezing
>> the sap heads to the trunk ... and finally to the roots when it
>> gets really cold. In short, it would seem the trees have a
>> proportional response to the cold temperatures. At least this
>> would explain antenna resistance observations at this location
>> with changing temperatures.
>>
>> Some good info on measuring field strength at:
>>
>> http://www.w1tag.com/FSM_RX-ANT.pdf
>>
>> http://www.w1tag.com/FSM_CAL.pdf
>>
>> http://www.w1tag.com/FSM_LOCS.pdf
>>
>> John and I both used identical setups and they worked
>> exceptionally well.
>>
>> Jay W1VD WD2XNS WE2XGR/2
>>
>>
>> ----- Original Message ----- From: N1BUG <[email protected]>
>> Reply-To: <[email protected]> To:
>> <[email protected]> Sent: 10/27/2018 6:34:57 AM
>> Subject: Antenna resistance (Re: LF: EbNaut Autodecoder 137490
>> Hz: EA5DOM received)
>> ________________________________________________________________________________
>>
>>
>>
Hi Luis, Markus, LF
>>
>> This is a very interesting topic for me too.
>>
>> My antenna resistance is very high, usually more than 100 ohms.
>> I think there may be a few reasons. There are trees near the
>> antenna which I presume to be lossy. My antenna ground system
>> is not tied into the house ground, in order to minimize 120 Hz
>> sidebands on my transmitted signal. When I tie the grounds
>> together the level of these sidebands increases almost 20 dB.
>> Also my RX antenna is more noisy when the TX antenna ground and
>> house ground are tied together. I suppose this is because more
>> noise is radiated from the TX antenna, then picked up by the RX
>> antenna which has its own small, isolated ground.
>>
>> Rain has no effect on the antenna resistance, but temperature
>> does. During summer there is little change, just minor
>> variations. But as the temperature drops and begins to approach
>> freezing, resistance begins to get lower. There is a big drop
>> right around freezing or few degrees below, but resistance
>> continues to drop with even lower temperatures, reaching
>> minimum on the coldest winter nights. Last winter I saw it
>> reach 40 ohms a few times. :)
>>
>> I am no expert but I guess the trees are partly responsible for
>> the high resistance and the drop with freezing temperature?
>>
>> Anyway I can run enough power to reach calculated 1W EIRP and
>> it's enough to be heard across that little pond. :)
>>
>> But here is a related question: I have calculated my EIRP to be
>> one watt using the measured resistance, antenna parameters and
>> current. But does this calculation include all the losses in
>> trees? If I had the proper equipment to make field strength
>> measurements at a distance, would I find that my EIRP is less
>> than one watt due to the tree losses?
>>
>> 73, Paul N1BUG
>>
>>
>>
>>> On 10/27/18 5:45 AM, Markus Vester wrote: Hi Luis,
>>>
>>>> Impedance drops a lot after rain
>>>
>>> This seems unusual. I have exactly the opposite effect here:
>>> Series resistance at 137 kHz is normally around 20 ohms
>>> (including 5 ohms for the coil). When it rains it get worse
>>> up to about 40 ohms, whereas best times are cold and dry
>>> winter days with ~15 ohms when the trees are solidly frozen.
>>> So I presume that the major contribution to my resistance
>>> are capacitively-coupled losses in vegetation and other
>>> nearfield environment. The ground connection itself (the
>>> "house earth" which is connected to electric grid PE, and
>>> also pipe systems for water supply and distict heating) seems
>>> to have a low resistance.
>>>
>>>
>>> My only explanation for lower wet resistance would be if one
>>> were using a separate radial system in the garden, whose
>>> connection to ground might be better when the lawn is soaked.
>>> But I don't think this applies to your setup at all.
>>>
>>> Best 73, Markus
>>>
>>> Von: VIGILANT Luis Fernández
>>>
>>> I'm struggling here with the antenna. Most of the time
>>> impedance rises and I hardly run 1A RF current
>>>
>>> Rainy weekend here. Impedance drops a lot after rain. Let's
>>> see how much current can drainLast time I could see almost 4A
>>> RF.
>>
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