I'm not an RF engineer (though I have a MSEE). RF people do everthing
in power, while baseband people think in terms of voltage. In the case
of using an ADC, of course you care about voltage so you don't clip.

I'm going to set up my large Wellbrook on Saturday and plug it in
directly to an oscilloscope with the 50 ohm terminator set. I'll let
you know what I get.

BTW, RF engineers use power meters a great deal. I have a HP 432B which
I'm sure will be able to get a reading from the Wellbrook. However,
unless some signal dominates, you really won't know the proper peak to
peak level due to an uncertain crest factor. Hopefully something will
show up on the scope. The power meter is broadband. I don't know the
lower limit, but the upper limit is 10Ghz.

Mark Zenier wrote:

In article ,
dxAce wrote:


wrote:

I suppose you could use the term untuned dipole, but I think most
people would interpret the term incorrectly. Functionally, both are
random lengths of wire.

If one has a dipole would it not be 'tuned' or 'resonant' on some frequency?

Resonance just means that it's operating at a point where the inductive
and capacitive reactances are equal. A non-resonant dipole is still
picking up the signal, it just presents it as an impedance that is
mismatched to the transmission line (or receiver).

Damn... I wasn't aware of that.

Thanks,

dxAce
Michigan
USA

In article ,
(Mark Zenier) wrote:

In article , dxAce
wrote:


wrote:

I suppose you could use the term untuned dipole, but I think most
people would interpret the term incorrectly. Functionally, both
are random lengths of wire.

If one has a dipole would it not be 'tuned' or 'resonant' on some
frequency?

Resonance just means that it's operating at a point where the
inductive and capacitive reactances are equal. A non-resonant dipole
is still picking up the signal, it just presents it as an impedance
that is mismatched to the transmission line (or receiver).

In general for RF circuits resonance means reactance is zero like you
said but antennas have a different spin on things.

The when the electrical "size" of the antenna is the same as a passing
EM wave the ability of the antenna as a "transducer" between a far field
and a local circuit is enhanced. Anyone that has used an antenna cut for
a band has experienced the best gain the antenna has to offer on that
band.

The best analogy I can think of is a tuning fork that will ring when
another of the same size is generating its resonant tone nearby.

--
Telamon
Ventura, California

Using 30ft of wire, square, in a Wellbrook ALA100, I get a whopping
120mVpp. It's much higher than I would have guessed. [Yes, the scope
was put in the 50 ohm position.] The signal is dominated by a local BCB
station at 740Khz, with a bit of 810KHz as well. I cranked the time
down way slow to get a fat band on the CRT to check the Vpp. It's the
occasional superpositon of sine waves that makes the vpp so high.

I have a selective level meter that I can try to use some other time to
see the energy at each frequency. I got it in a bid lot years ago (I
wanted something else in the lot), so I don't even know if it works. [I
fact I got two and gave one away to save space!] I'd have to run a
signal generator into it and see if it is trustworthy.

Anyway, that should give you a starting point. It's kind of obvious now
why the 7030 switches in some sort of filter once you go about the BCB
(technically the old US BCB, before it was extended.] The signal levels
above AM BCB must be at least an order of magnitude lower, maybe more.
If BCB is not in your target, I'd suggest a high pass filter before
your amplifier.

Unless you have a spectrum analyzer handy, you might want to consider
getting a selective level meter for your project, if only to very your
measurements.
http://cgi.ebay.com/ws/eBayISAPI.dll...sPageName=WDVW
They don't go for much money since they supply is huge. They were used
in telco years ago but are not required anymore (different technology
or something like that.)

Telamon wrote:

The when the electrical "size" of the antenna is the same as a passing
EM wave the ability of the antenna as a "transducer" between a far field
and a local circuit is enhanced. Anyone that has used an antenna cut for
a band has experienced the best gain the antenna has to offer on that
band.


I don't know if gain is the right word. Is 'least loss' more appropriate?





mike

m II wrote:

Telamon wrote:

The when the electrical "size" of the antenna is the same as a passing
EM wave the ability of the antenna as a "transducer" between a far field
and a local circuit is enhanced. Anyone that has used an antenna cut for
a band has experienced the best gain the antenna has to offer on that
band.

I don't know if gain is the right word. Is 'least loss' more appropriate?

Gain would be appropriate when comparing one antenna to another, or to an
isotropic source.

dxAce
Michigan
USA

Hello John,

No preamp is necessary or even desirable. I highly recommend a rotator.
This is the only "option" that is desirable. An inexpensive Radio Shack
one will do. I use their 15-1245 which sells for about $75. You will
also need their 20-gauge three-wire rotator cable (make sure you get
exactly that only - their salespeople are not always familiar with it).
It sells for about $15 for 100 feet.

The antenna comes with two spare fuses (it should be rarely, if ever,
necessary to replace the fuse) and a Stancor STA-300R AC-adapter for
the US. Should you wish to purchase additional "backups," Allied
Electronics sells them at reasonable prices.

Best,

Joe

You must have missed my post on the signal level I got out of my
ALA100. John wants 5.5dbm,or nearly 1.2Vpp if I did the math right. My
ALA 100 produced 120mVpp, so a gain of 10 is needed at the least. I had
30ft of wire in my loop, so the signal strength is probably larger than
an off the shelf Wellbrook loop.

What did you measure from your loop to lead you to the conclusion no
extra gain was needed?

dxAce wrote:

I don't know if gain is the right word. Is 'least loss' more appropriate?


Gain would be appropriate when comparing one antenna to another, or to an
isotropic source.


There may be something that resembles an isotropic source, but these guys claim
an appropriate antenna is just hypothetical.

http://radioworks.com/nhpant.html


The idea of 'gain' is still troubling, though. I can see a tuned circuit
building up energy if there were no resistance in the circuit. With the
minuscule voltages available in many shortwave radio signals, there isn't much
to spare. Getting the antenna to lose as little as possible is an easier concept.

I'll have to do some research on this.




mike

In article ,
dxAce wrote:

m II wrote:

Telamon wrote:

The when the electrical "size" of the antenna is the same as a passing
EM wave the ability of the antenna as a "transducer" between a far field
and a local circuit is enhanced. Anyone that has used an antenna cut for
a band has experienced the best gain the antenna has to offer on that
band.

I don't know if gain is the right word. Is 'least loss' more appropriate?

Gain would be appropriate when comparing one antenna to another, or to an
isotropic source.

Most if not all practical antennas have directivity and therefor gain. I
was not being specific as to what the gain was because I was making a
general statement about antennas so I did not make the reference.

--
Telamon
Ventura, California