pse remove the nl newsgroup from this discussion.


John Doty schreef op 29-6-04 :
Richard Clark wrote:

Antennas have no capacity to reduce Signal to Noise ratios except by
virtue of narrowing lobes to eliminate noise by placing it in a null
(if that is in fact a viable option either in the sense of having a
null, or having a null to a noise source that is not on the same
meridian as the signal of interest).

Not true. You are making the assumption that that the antenna only picks up
radiated modes. 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.

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.

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.

In fact, the presumption there are unique reception
antennas that are more suitable than their transmission cousins is
simply the artifice of my aforementioned advantage of the RF Gain
control. It has been long established (through the simple act of
purchase power) that receivers have far more gain available than
needed except for the worst of antenna designs (and that has to be an
exceptionally vile design).

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

This does NOT demonstrate some illusion of superior
receive antenna design; rather it is more smoke and mirrors as an
argument. Inverting the argument, if you had a full sized antenna for
that band, you would only need a galena crystal and cat whisker to
power your hi-Z headset. For DX you would only need a $5 AF
amplifier. The smaller antenna clearly needs more dollars expended to
offset the debilities of the poorer efficiency.

Sensitivity is the cheapest, easiest virtue to put into a receiver.
Essentially all modern receivers have plenty. Indeed, the cheap ones often
overload when presented with an efficient antenna: you have to spend the
dollars to be able to handle the big signals!

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

The specious argument
is tailored for the technically effete who would rather push a credit
card across the display counter than build their own cheap solution.
Take heart that this not simply a cheap shot, there are as many Hams
who don't know which end of the soldering iron to pick up either.

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