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-   -   Let's design a short antenna just for fun (https://www.radiobanter.com/antenna/208996-lets-design-short-antenna-just-fun.html)

John S November 8th 14 05:37 PM
Let's design a short antenna just for fun
 
On 11/8/2014 11:11 AM, wrote:
Lostgallifreyan wrote:
After posting, I remembered one important one, electrical resistance. That
might strongly reject austenitic stainless as a choice, if the antenna is
small or otherwise makes low resistance critical.

On the other hand its high strwength to weight ratio might be another strong
plus to add to any others, it can save nasty accidents with underestimating
wind loads, breaking strains and such when testing a new build. (For wires,
I'm less sure about towers, I haven't learned enough to make suggestions for
materials and structural forms and dimensions for those).

The resistivity of stainless steel is about 35 times that of copper and
about 20 times that of aluminum.

This will make a big difference for a wire antenna, but less for one
of fat tubing or rod, however stainless tubing is both heavy and expensive.

It would all be a bunch of engineering trade offs that would depend on
what one wants to accomplish.

If it happens that your brother-in-law can get you stainless at a good
price...

Thanks for that, Jim. I don't know where I got the idea that SS was
close to that of copper.

[email protected] November 8th 14 06:12 PM
Let's design a short antenna just for fun
 
Lostgallifreyan wrote:
wrote in :

The resistivity of stainless steel is about 35 times that of copper and
about 20 times that of aluminum.

This will make a big difference for a wire antenna, but less for one
of fat tubing or rod, however stainless tubing is both heavy and expensive.

It would all be a bunch of engineering trade offs that would depend on
what one wants to accomplish.

If it happens that your brother-in-law can get you stainless at a good
price...


I have no brother-in-law. :) But I usually have a bit of stainless steel
around.

Ok, so no high current transmitters unless some sturdy mechanical design is
used. What about QRP? I understand that high efficiency and a large antenna
is best for very low power transmitters, and given the lower currents
involved, would I^2R losses be low enough to be pretty much as viable as for
reception? This strays from the short antenna subject, but I'm interested
enough to ask anyway.

Loss is loss, whether receiving or transmitting.

The only difference is loss on receive can mostly be made up for by
an amplifier in front of the receiver, a technique used in the small
directional loops often used for receive on the lower bands. The
directionality and low bandwidth of the loop reduces the noise and
signals from other directions.

Can't hurt to try and the free demo version of EZNEC can be used for
simple antennas and will show you the effects of using stainless.



--
Jim Pennino

Lostgallifreyan November 8th 14 07:32 PM
Let's design a short antenna just for fun
 
wrote in :

Loss is loss, whether receiving or transmitting.


Yes, but as you say, if the SNR is ok, then an amp can help on receive. What
I wondered was if the losses being a lot lower with lower current (I^2R
suggesting a significant inverse-square reduction as current falls), whether
a low power transmission justifies 316 stainless where a higher current would
not. To put it another way, if there is, is there some crude rule to suggest
a current (transmitter power) that makes it better to go with copper? (I'm
assuming a half-wave dipole for now, not the small antenna the subject
indicates).

I'm not ready to trust my use of NEC yet, I've been visiting people about
amplifier repairs and such, my sight's bad enough that I'm limiting my
computer time deliberately for a while to see if that helps.

[email protected] November 8th 14 09:31 PM
Let's design a short antenna just for fun
 
Lostgallifreyan wrote:
wrote in :

Loss is loss, whether receiving or transmitting.


Yes, but as you say, if the SNR is ok, then an amp can help on receive. What
I wondered was if the losses being a lot lower with lower current (I^2R
suggesting a significant inverse-square reduction as current falls), whether
a low power transmission justifies 316 stainless where a higher current would
not. To put it another way, if there is, is there some crude rule to suggest
a current (transmitter power) that makes it better to go with copper? (I'm
assuming a half-wave dipole for now, not the small antenna the subject
indicates).

Loss is a multiplicative thing; 50% of 100W is 50W, 50% of 10W is 5W.

As for a 1/2 wave dipole in particular, unless the stainless is very
thin it is unlikely to make enough difference to notice without test
equipment.

I'm not ready to trust my use of NEC yet, I've been visiting people about
amplifier repairs and such, my sight's bad enough that I'm limiting my
computer time deliberately for a while to see if that helps.

Have you thought about purpose dedicated glasses?

I have three pair; one for everyday, one for target shooting, and yet
another for flying.


--
Jim Pennino

Lostgallifreyan November 9th 14 10:05 AM
Let's design a short antenna just for fun
 
wrote in :

Have you thought about purpose dedicated glasses?


Yes, that's as afr as I got so far. :) Thought about it... Thing is, there's
a kind of shark's fin defect in horiziontal lines, seen in one eye. For at
least two years my brain mushed over that one, it took some time before I
could repeatedly resolve it and have anythign to tell a doctor about.
Whatever this is, it's not just focus. Courage to see doctors can only come
from conviction I should do so, and I'm maybe at that point by now.

Lostgallifreyan November 9th 14 10:08 AM
Let's design a short antenna just for fun
 
wrote in :

As for a 1/2 wave dipole in particular, unless the stainless is very
thin it is unlikely to make enough difference to notice without test
equipment.


Ok, that works for me. :) At some point when I've learned enough about NEC
for it to be fast to set up small tests on a whim or idea, but in my location
there might be so many buildings that reflections could hamper any attempt to
get results to match any simple model of any kind. In short, it it becomes
easier to do empirically than to model, I do that.

Lostgallifreyan November 9th 14 10:11 AM
Let's design a short antenna just for fun
 
wrote in :

Loss is a multiplicative thing; 50% of 100W is 50W, 50% of 10W is 5W.


In other words, proportional no matter what the actual power.. ok, I can live
with that. :) Is the same in much of optics, and I forgot the direct
relation.


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