Richard Clark wrote:
On Tue, 29 Jun 2004 15:34:42 -0600, John Doty
wrote:


You are making the assumption that that the antenna only picks
up radiated modes.


I am making no such assumption and all following commentary does
absolutely nothing to separate the concerns of SWLers from Ham
activity.


Non-radiated electromagnetic modes are also
troublesome, particularly common mode on the transmission line. This
tends to be the way that locally generated noise from household gadgets
gets into an antenna system.

Consider a lamp dimmer that generates 10 mW of RFI, which rides out in
common mode on the mains, finds its way to the power cord of your
transceiver, rides out on the feedline to the antenna, and then couples
back through differential mode to your receiver input. That's not a very
efficient coupling path, so suppose it has a loss of 60 dB. You'll still
get 10 nW to the receiver. This is a lot: even if it's spread over 30
MHz, it's still 10 uV in a 6 kHz channel. That's S6 on my Drake R-8, a
very serious quantity of noise.


Let's work with exactly that scenario you offered.

S6 (Calibrated) on my Drake TR-7 is -88dBm - so close to your 10µV to
be indistinguishable. My TS-430 varies from -80dBm to -73dBm. There
is no calibrated S-Meter for my DX-440, but for a $200 SW set, its
sensitivity is -90dBm for a full scale meter indication (about 7dB
range from top to bottom).

All very well and good. Now if we regard this speculation of 10mW (it
is, after all, the epitome of a wild ass guess, isn't it?); then,
let's reverse engineer that 10nW product from 6kHz buckets over the
range of 30MHz to find 50µW which is 23dB below the original power
presumably suppressed 60dB. Well, I have either pencil-whipped you,
or you me, or each other - the numbers don't add up.

10 uV into 50 ohms is 2 pW, not 10 nW (E^2/R). 2 pW = -117 dBW = -87
dBm. Multiplying by 30000/6 = 5000 buckets makes 10 nW or -50 dBm.
Cancel the assumed 60 dB loss and I get +10 dBm, or 10 mW. The numbers
add up fine.


However, if I return to the original "problem" of noise derived from
household sources; then that is also something I have closely
measured.

Across time, frequency, antennas, and known noise sources I have found
it as low as S1 for my longwire (an antenna supposedly unused by Hams)
to as high as S7 (for that same longwire). My loops, dipoles and
verticals hardly fell outside of this range to present any gilt-edge
design.

Just because you couldn't doesn't mean others can't. Look at the rest of
the articles on the BADX site. Taking steps to minimize common mode
coupling has worked very well for me, and many people tell me it works
for them too.

You might also find the articles at http://www.qsl.net/wa1ion/
interesting, especially the one entitled "Another Look at Noise Reducing
Antennas". Mark's antenna designs are generally useless for
transmitting, but they make superb MWDX receiving antennas.


With every circuit in the house broken (operating battery power in the
dark), average noise level was either S2 for a vertical, or S1 to S3
for a loop (rather upsetting the voodoo of loops being quiet and
verticals being noisy). When I returned power to the house by stages,
I insured every opportunity of injecting noise by setting dimmers to
their worst position (about 50%). In the low bands, I suffered as
much as S8 noise levels with an average of S5 when the house was full
lit (also including fluorescents) and all noise sources adding to the
cacophony of reception. This was for a loop antenna.


On the other hand, if your transmitter puts out 1 kW, 60 dB of loss
means it only delivers 1 mW of RF to the dimmer, an amount unlikely to
interfere with its operation. Reciprocity does not mean *consequences*
are symmetrical.


This effect of reciprocity has been reported so frequently in this
group so as to negate your premise. We have many queries for how to
solve this problem.


To this point, you have not offered any particularly receive dominated
issue that is not already a heavily trafficked topic with transmission
antennas.

A deep, steerable null can be extremely useful for reception, but its
not generally useful for transmission.


This really goes off the deep edge. Barring loss introduced for the
sake of jimmying the logic, transmitters AND receivers enjoy the GAIN
derived from the introduction of a null not otherwise part of the
characteristic. This is a commonplace of theory and practice. Where
ever you can design or contribute to a null; then this must of
necessity result in an increase in signal outside of its region.
These are all commonplace observations discussed here that are
observable for either Ham or SWL operations. There is NO differential
offered in these observations that separate SWL from Ham activities.

Certainly there is. A narrow null takes little power from the pattern:
you get little gain by putting that in a broad lobe. For example, an
elementary dipole has, theoretically, infinitely deep nulls yet it only
has about 2 dBi gain. Now consider a phased array: small phasing errors
have little effect on the gain, but they can have a large effect on the
null depth.

When transmitting, you're generally interested in putting the power in
the right place, but when receiving you're often more interested in
avoiding picking up power from the wrong place. These considerations are
only weakly related.



Such examples of small loops used for MF are proof positive how poor
an antenna can be, and the RF gain knob resurrecting its pitiful
efficiency.

But for MWDX reception, efficiency simply isn't an important virtue.


I believe I have said that at least 3 to 5 times already.


Gain is cheap. What matters is the steerable nulls. An efficient
*steerable* MW antenna is enormous and expensive.


Who needs an efficient MW antenna?

People who transmit, of course!

Speaking of strawmen, have you ever actually tried DXing with a crystal
radio?


Sure, what is so remarkable about that? Beyond this simple design,
ever hear of a super-Regen receiver? You don't need to spend half a
kilo-buck to get the same sensitivity and filtering is dirt cheap.

Sure. Used them. Selectivity is *lousy*. For SW, I've gotten better
results with a plain regen. Still, my Drake R-8 is better for DX, my
Sony ICF-SW100 travels easy, and my Stromberg-Carlson 58-T sounds
wonderful, so I haven't played with a regen in quite a while.

How about Q-multipliers? All such topics barely spread the wallet as
much as the illusion of more buttons make a better rig.

I have a Heathkit Q-multiplier I built in 1965. It's pretty good at
nulling out unwanted carriers, but in peak mode the shape factor of a
single resonance is pretty poor. At the moment I don't have a working
receiver it's really suited to, but I have a Halli S-40 one of my
in-laws gave me, and one of these days I'll find the time to repair it
(it's in really poor shape). The S-40 could probably use a Q-multiplier
once I've got it in working order.



I love designing and building antennas: applied physics is fun. But it's
good engineering to go with the strengths of your technology. For my
inverted-L's, I spend a little efficiency (4 dB or so) to get octaves of
effective bandwidth, something that is perhaps of little use to hams,
but is very useful to an SWL in conjunction with the frequency agility
of a modern receiver. 4 dB of efficiency loss is of negligible
consequence at HF and below if your receiver has a decent noise figure.


As I pointed out to Yahoo, if you choose to cripple yourself, then
slide on over to the shoulder and enjoy kicking up dust and rocks as
you travel down the road. a 4dB loss for an inverted L (hardly a SW
invention) is far too simple to remedy to make its suffering a boast
of martyrdom. It is a strange argument to offer that you can't afford
a $20 solution for your $500 set and $2 worth of wire.

How would you undo that 4 dB loss without loss of bandwidth? I'm hardly
boasting of martyrdom anyway: a broadband inverted L is a fine general
purpose receiving antenna.



I've never seen mention of this efficiency/bandwidth tradeoff in the ham
literature,


You haven't looked. Either contrived, wholly fictional, or accurately
represented, it is part of the stock in trade for selling antennas.
In this group, I would wager its discussion consumes more bandwidth
than bragging about how many QSL cards have been pasted to the wall.

Examples?

but it's not hard to find in the professional literature.
For details of a specific calculation, see:

http://anarc.org/naswa/badx/antennas/SWL_longwire.html

-jpd


It would do you well to note that this "professional" whom you rely
upon, John Kraus, is one of the most notable Ham Radio Operators
frequently acknowledged and referred to here.

He knew a great deal about the full range of antenna designs and
applications, not just ham radio. And he sure knew his physics.


Do you or others have any actual differentiable discussion, or is this
simply an outlet for appoligia for why it isn't worth the strain to
lift a soldering iron when you can bench press a credit card?

When a soldering iron is the best tool to get the job done, I lift a
soldering iron. The Stromberg-Carlson was a "bare chassis" restoration:
most of the resistors were 50% off value, the paper capacitors were
leaking, the electrolytics were dry, and one of the RF coils was open
(disassembling and reassembling the coil turret was a real pain). Still,
it was worth the work to get that wonderful sound.

The NASA certified techs I work with professionally tell me I'm pretty
good with a soldering iron, considering I'm a physicist. From them,
that's high praise :-)

-jpd