On 11/4/2015 8:06 PM, Jeff Liebermann wrote:
On Wed, 4 Nov 2015 01:27:16 -0500, rickman wrote:
On 11/4/2015 12:41 AM, Jeff Liebermann wrote:
On Tue, 3 Nov 2015 03:37:36 -0500, rickman wrote:
Sorry, but I need to bail out of this interesting discussion for about
a week. I just landed another satellite dish repair job and need to
steal some time.
Yeah, me too.
I'm back. I got a one week delay. I get to do the dishes next Thurs.
However, I still need to reduce my usenet time in order to get a few
important things done. (If I did everything I promised to do, I'd
never get anything done).
It is not very useful to get a 2.5% improvement. That's the bottom line.
Yes, but silver plating looks cool and will probably sell a few more
overpriced antennas. I guess the generic version should be polished
copper coated with Krylon, while the "pro" version might be silver
plated and coated with Krylon. Sorry, but no "Monster Cable" model in
2% gold is planned. Besides, at the high end, diminishing returns
becomes a fact-o-life. For a 2.5% improvement, you get to pay 50%
more. Seems fair to me.
I believe gold is not as good a conductor as copper. The rank is
silver, copper, gold, aluminum with silver only 5% better than copper
which is mitigated to 2.5% with the skin effect.
I'm looking at aluminum because of the cost and the weight, but
noticeably less with aluminum.
You are now analyzing receiving antennas. That's a gear shift. I've
been discussing transmitting antennas. Big distinction.
Receive is my main area of interest. I'm trying not to do anything
that will preclude its use as a transmit antenna. At QRP levels
(5watts), the distinction isn't that big. The fun starts at 50 watts
and up. From the standpoint of construction, the big difference is
that the tuning cap has to handle high voltages and that the loop
needs to survive high currents.
Receive and transmit are opposed goals for optimization. A high
radiation resistance means some of your received signal is radiated
again. A low radiation resistance lowers the transmission efficiency.
The other issues both have in common, but it is easier to optimize a
receive antenna than a transmit antenna. I have seen more than one have
use separate antennas for each.
Incidentally, this is one reason why I can't directly answer some of
your questions and why I seem to be drifting in topic. I'm following
my own reading and tinkering, not yours.
It makes a huge difference. No one makes a transmit antenna with
multiturns and small wire which are both perfectly ok for receive. Here
are the key equations for receive antennas...
In general the receive voltage relates to the various parameters
assuming the radiation resistance is small -
L ∝ r * ln(r) * N2
R ∝ r * N
Q ∝ N * ln(r)
V ∝ r² * N * Q * ln(r)
V ∝ r² * N² * ln(r)
l ∝ r * N * ln(r)
V ∝ l² * ln(r)
So maximizing signal strength means maximizing the total length of the
coil independent of the number of turns other than the small effect from
ln(r). Smaller loops with more turns is nearly as good as larger loops
with fewer turns. Not so for transmitting antennas because the
radiation resistance which needs to be than the ohmic resistance. A
large radiation resistance can hurt the Q relative to what you get with
a receive antenna.
Consider using two antennas where the receive antenna has a lot more
length. No high voltages or currents are used so the components can be
much less costly. A simple air cap with a standard wiper or bearing
connected rotor can be used.
Hmmm... if that's correct, it might be useful for my quest for the
worlds smallest practical HF loop.
Xmit and receive put very different requirements on the antenna. Which
do you wish to optimize?
Initially, just receive performance. Once that's working and
understood, the tuning cap and loop construction can be beefed up to
handle the voltages and current levels needed for transmit.
What power level/range are you shooting for?
Initially QRP (5 watts). Next about 50 watts (digital modes).
Eventually, 150 watts (SSB). These can be 3 different models, with 3
different capacitors and 3 different mechanical designs. After some
tinkering, I know what it takes to make something that works in
transmit. What I don't know is how small I can make the loop and
that's what I'm initially working on calculating and testing.
An all too common problem is that the tuning changes between trnansmit
and receive. If I can't cure that, I'll probably need remote antenna
tuning, motor drive, uP control, etc.
Are you talking about self heating effects?
I seem to recall some errors were reported, but I don't recall them
being of any consequence.
You haven't indicated if it's your model. I uploaded it to:
http://802.11junk.com/jeffl/antennas/magnetic-loop/Antenna_trans_LTspice/Antenna_trans_loop.asc
Is this the latest? This is what it produces:
Circuit: *
C:\blah-blah\jeffl\antennas\magnetic-loop\Antenna_trans_LTspice\Antenna_trans_loop.asc
Number of points per octave reduced from 3000000 to 19545.
Multiply defined .measure result: max
Each .measure statement needs a unique result name.
Date: Wed Nov 04 16:49:57 2015
Total elapsed time: 0.266 seconds.
Yes, I wrote the simulation with help from a variety of sources. The
above is not really an error. Just reduce the number of points used. I
don't recall how that is set, but much of it is parametrized.
I'm not sure what is up with the MAX error report. That sounds like a
problem with a line continuation.
I have no idea how it will work on thinwall sections.
That's a big deal. It needs to work with thin tubing.
Time permitting, I'll try it on whatever aluminum tubing I can find. I
have an aluminum ladder than could use some reinforcing, so I'll get
some practice. I'll probably have to use propane as oxy-acetylene
will probably burn a hole in it.
I have a friend who is a great welder, but he is older than myself and
doesn't spend much time in the shop these days. I visited him today and
we just hung out in the workshop and talked about stuff. He is trying
to improve his TV reception by adding another antenna on the same pole
and connecting the two together through one preamp. I told him if the
antenna are close together they may interfere and using one preamp is
likely to be a problem. He was not happy...
I'm happy with the idea of soldering.
"How to Solder Aluminum Thin Wall Tubing"
http://www.ehow.com/how_6069853_solder-aluminum-thin-wall-tubing.html
I will look into that.
--
Rick