[TowerTalk] Inv v lengths

[Lux, Jim]

On 9/21/22 2:04 PM, Jim W7RY wrote:
> No kidding Gregg!
> Do we really have to make rocket surgery about this?


Not if all you're doing is throwing up a dipole and want a "reasonable" 
match - I just make it close, and use a tuner to take out the small error.

But if you're doing a phased array (like a 4 square) then you might care 
a bit more, because the F/B and null depth is going to depend on small 
changes.


And, to make a direct rocket example - I'm working on a project (SunRISE 
mission) with 6 spacecraft orbiting the Earth just above GEO to make 
radio observations of the Sun. We have 2 dipoles, a bit more than 5 
meters long overall, on each spacecraft to receive from 0.1 MHz to 23 
MHz.  We care *a lot* about the exact impedance vs frequency for two 
reasons:

1) the receiver is a high Z preamplifier with shunt C (parasitic), so 
the circuit looks like a capacitive voltage divider.  It's not practical 
to test the antenna on a range (that 3km wavelength makes things tricky).

2) we will "see" about half the Earth, so all those FM and TV stations 
are up near the resonances (30 and 90 MHz) and anti-resonances (60 MHz, 
120 MHz) of the antenna.  We don't want to overload the preamp.

So for this kind of thing, yeah, we DO care about small effects in the 
model.  And it's really useful to "know" what the changes are if the 
antenna booms deploy to 2.7 meters instead of 2.6 or 2.8 meters.

Another "space" application of modeling, where we care about insulated 
wires (and soil properties) is for a proposed radio observatory on the 
far side of the Moon.  There, the dipoles are 100m long and are 
insulated wire laid out on the surface of the Moon.




>
> 73, Jim W7RY
>
>
> On 9/21/2022 1:03 PM, Gregg Seidl wrote:
>> Goodness. It's not that hard. My experience is 468/ freq. Cut it a foot
>> longer and put it up. You might have to fold some back on the 
>> insulators.
>> Just try it.
>>
>> Gregg. K9KL
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>