On 11/6/2014 10:52 AM, rickman wrote:
On 11/6/2014 11:08 AM, John S wrote:
On 11/5/2014 7:16 PM, rickman wrote:
On 11/5/2014 7:28 PM, wrote:
I started to do some modeling on a short antenna for 160M and got what
I think are interesting results.
I will post those as soon as I get a chance to write up all the data.
All this stuff for short antenna is in the context of transmissions,
right? For receiving a short antenna is at a disadvantage, no? I seem
to recall a parameter called "effective height". For loop antenna it
pertains to the signal collected irrespective of the actual dimensions
of the loop. For other types of antenna I assume this is not the same
and does relate directly to the length of the antenna. Is that correct?
I ran a simulation to confirm that the received signal is some function
of the length of a wire antenna. My model was a 6 foot zero-loss wire 10
miles from the source with a load of 1000 ohms. The frequency is 1MHz.
Wire length Volts received
6' 0.001499
12' 0.005408
So, it appears that doubling the length of a short antenna captures
about 3.6 times the signal.
Is this what you wanted to know?
That is a nice experimental verification. I guess I figured this is the
sort of thing that there would be an equation for. A loop antenna has a
simple equation defining its effective height (ability to convert the
field to a voltage). I expect there is a similar equation for each
antenna type.
I have not used an equation. I used EZNEC. You can get a free trial
version. It has limitations, but no time limit IIRC.
I guess the point is that for receiving it is important to match the
size of the antenna to the signal to receive the maximum power. Or is
there something equivalent to the matching network that would equalize
the power received? In your example you said you used a 1000 ohm load.
Is there a way to improve the signal from the shorter antenna?
In receiving, it seems that size matters when it comes to small antennas
like we are discussing.
If you could increase the (receiver) input impedance you will get more
voltage. No matter what you do, you cannot increase the power received
except by refining your system. The (volts/meter)^2 is fixed. It is up
you to capture the available signal. And, I think it will take somewhat
heroic efforts at your frequency of interest.
BTW, have you seen the extremely tiny ferrite rod antennas used in the
so-called Atomic wris****ches?