Yikes! I have received several personal comments since I posted the
results. I guess some folks don't want to comment publicly.
I did the test since the arguments being made weren't proving anything.
RF current (as does DC and low frequency current) always takes the path
of overall least impedance. This means the currents spread out as in a
strip line on a PCB where most of the current in the ground is under the
transmission line but a considerable amount spreads out since the ground
has a finite RF resistance. Likewise in coax some of the shield current
gets to the outside of the coax which means the coax does radiate a
little. Of course good hard line reduces this number to a minuscule
amount but it still is not zero.
A fun test to do is to calibrate a VNA with a 10' piece of coax that is
terminated in the proper impedance at the end and then bring the end of
the coax back to the VNA and connect the center of the terminated coax
to the VNA case (ground) where the coax exited the VNA. This would
appear to be a short BUT at frequencies above about 5MHz the SWR will be
less than 1.5:1. Put some ferrite cores over the coax and this
frequency drops more. All RF transmission line transformers and current
baluns rely on this to work.
Cool stuff but not theory. It is real.
Larry, W0QE
David Cutter wrote:
> The current is taking many paths across the surface of the disc to get
> to the other side. Not sure what this means, but if you used a strip
> to reduce the number of paths, that might demonstrate something... I
> don't know what, but it would be an interesting comparison.
>
> David
> G3UNA
>
>
>
>> Here are the results of the experiment:
>>
>> 1.) Dipole was made with #10 bare copper wire each side 12.0" long.
>> 2.) Disks are 2.02" diameter x 0.01: thick copper with hole in center
>> for #10 wire.
>> 3.) 8ft. of RG-316 coax with 3 common mode chokes (Z>300 ohms at 200
>> MHz)was calibrated at the far end in order to measure actual dipole feed
>> point impedance.
>> 4.) Tests were done in my basement with at dipole height of about 3.5'
>> and 4-5' of clearance to other objects.
>>
>> Test #1: Bare dipole
>> Fres = 236.20 MHz, Z = 71.5 + j0 (468/length = 234MHz)
>>
>> Test #2: Dipole going thru center of disks and soldered with disks
>> placed 4.0" both sides of the center of the dipole.
>> Fres = 223.55 MHz, Z = 65.7 + j0
>>
>> Test #3: Dipole going thru center of disks and soldered with disks
>> placed 8.0" both sides of the center of the dipole.
>> Fres = 199.01 MHz, Z = 44.3 + j0
>>
>> Test #4: Dipole soldered to top edge of each disk with disks placed 8.0"
>> both sides of the center of the dipole.
>> Fres = 196.89 MHz, Z = 43.9 + j0
>>
>> Conclusions:
>> 1.) The difference in frequency between tests #1 and #2 or #3 and #4 do
>> NOT allow for the dipole length to be increased by the extra 4" it would
>> take if the current went around the disks.
>>
>> I have pictures if anyone questions my setup etc.
>>
>> 73,
>> Larry, W0QE
>>
>
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