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Don't you need _very_ well matched JFETs
to ensure drain current balance in that circuit?

Could this be avoided by giving each JFET source
an independent primary and 330 ohm resistor,
then linking the "centre-tap" with the capacitor?

If you were making a differential amplifier for instrumentation
purposes, yes. But in this application you are making a pair of source
followers, and the amount of error in using an unmatched pair in such an
application would hardly be noticeable. However, if you want to go to the
extreme of precisely matching (meaning "very well") devices down to 0.01%
you may feel free to do so.

Your second suggestion would be sufficient to overcome the adversity of
using unmatched FETs if you feel that it's necessary to be that exact. I
had thought of doing so, but the application is not that demanding since the
load impedance presented by the 2N5109 as seen through the transformer is
quite high.

Chris

,----------------------. High Performance Mixers and
/ What's all this \ Amplifiers for RF Communications
/ extinct stuff, anyhow? /
\ _______,--------------' Chris Trask / N7ZWY
_ |/ Principal Engineer
oo\ Sonoran Radio Research
(__)\ _ P.O. Box 25240
\ \ .' `. Tempe, Arizona 85285-5240
\ \ / \
\ '" \ IEEE Senior Member #40274515
. ( ) \
'-| )__| :. \ Email: christrask@...
| | | | \ '.
c__; c__; '-..'>.__

Graphics by Loek Frederiks

Hi Andy,
Yes, I agree. It makes sense that that efficiency would be a power in -
power out ratio. I was wondering how to compare it, though. E.g.,
what would be the efficiency of a plain old dipole up 30 feet in my
backyard? If the efficiency for that arrangement were, say, twice that
of a proposed rigid loop as calculated via the KI6GD program, I would
have second thoughts about spending the money to build such a loop (or I
would increase circumference up to 33 % or increase pipe diameter up to
the limit of my wallet.)
As I play with the program, using "reasonable" copper diameters and side
lengths (reasonable to me), I come up with efficiencies in the 25 to 40
% range.
Thanks,
Dave

--- In loopantennas@..., Andy <Andy.groups@...> wrote:

Also, what is the LoopCalc program comparing to when it states that a
certain configuration is , e.g., 51% efficient. Compared to a

standard

dipole?

I think all antenna efficiency programs use a resonant dipole

in free

space as the standard for antenna efficiency.

Actually, as far as efficiency is concerned, the reference is ideal or

no

loss. Efficiency, unlike gain, is not compared to a dipole or anything
else. 100% efficiency just means that all the power you feed into the
antenna, gets turned into radio waves and none is lost in the form of

heat

(resistance losses). 50% means half your input power is lost as heat

and

never gets out.

After dealing with the power lost to heat, then what's left over is

affected

by the antenna's gain ... which IS compared to a reference antenna

(dipole

or isotropic), and is a function of direction (since antenna gain is

all

about concentrating signals in some directions while sacrificing signal

in

other directions).

Andy

Also, what is the LoopCalc program comparing to when it states that a
certain configuration is , e.g., 51% efficient. Compared to a standard
dipole?

I think all antenna efficiency programs use a resonant dipole in free
space as the standard for antenna efficiency.

Actually, as far as efficiency is concerned, the reference is ideal or no
loss. Efficiency, unlike gain, is not compared to a dipole or anything
else. 100% efficiency just means that all the power you feed into the
antenna, gets turned into radio waves and none is lost in the form of heat
(resistance losses). 50% means half your input power is lost as heat and
never gets out.

After dealing with the power lost to heat, then what's left over is affected
by the antenna's gain ... which IS compared to a reference antenna (dipole
or isotropic), and is a function of direction (since antenna gain is all
about concentrating signals in some directions while sacrificing signal in
other directions).

Andy

Sam Morgan wrote:

using hamcalc
for 1.9mc
I come up with
a ~15 turn coil of RG-58
on a 3.5" coil form
using ~13' of coax
and the 80m loop needs to be shortened by ~ 13'

These lengths sound short to me.

I have had other replies that mentioned
using an open 1/4 wavelength stub
using RG-58 that would be ~ 128'
am I on track here?
would either way work?
is one way more narrow banded than the other?
meaning if I made this for 1.9mc
would it still work at 1.8 or 2.0mc?

I think a 1/4 wave shorted stub would be wider band than a half wave open stub. The open stub is also prone to arching from the high peak voltage at the open end during transmitting.

K4DTT@... wrote:

_ () Here is a site that describes the trap thing very well. I cannot find the QST article that I was thinking of when I first posted to you. Good luck and great fun.

thanks for the info, defiantly a big help.

using hamcalc
for 1.9mc
I come up with
a ~15 turn coil of RG-58
on a 3.5" coil form
using ~13' of coax
and the 80m loop needs to be shortened by ~ 13'

I have had other replies that mentioned
using an open 1/4 wavelength stub
using RG-58 that would be ~ 128'

am I on track here?
would either way work?
is one way more narrow banded than the other?
meaning if I made this for 1.9mc
would it still work at 1.8 or 2.0mc?
--
GB & 73's
KA5OAI
Sam Morgan

I don't know about the stub but I am sure the trap will cover the band as it
seems to be rather low Q. I haven't tried it.

73 Glenn K4DTT



*** See what's free at .

Rf = free space impedence (small for small loops)

Rs = skin effect resistance (depends on bulk resistivity, usable skin area,
frequency)

Efficiency = Rf / (Rf + Rs)

You can see as Rs approaches zero efficiency approaches 1 and as Rs equals
Rf efficiency is halved.

Note that in addition to skin effect other resistances could be in an actual
implementation such as the resistance of joints to construct the loop,
connection to the capacitor and the implementation of the capacitor itself.

All those would add to Rs and further degrade the efficiency.

73

jim ab3cv

seems i screwed up my efficiency calculation. but the thought was right...

i'll post corrected version in a few minutes.

jtm

Thanks Jim, Todd and Dave. Good points. Helps my understanding a lot.
Dave G.
N2chi

--- In loopantennas@..., "Jim Miller" <jim@...> wrote:

self resonance is the problem above a particular diameter. you can

actually

see this as you watch the capacitor needed for a given frequency as you
increase the size.

efficiency is the ratio of the skin effect impedence over the sum of

the

skin effect plus free space impedence (which is pretty small.) note

that the

efficiency doesn't take into account any joints which will reduce the
efficiency even further.

73

jim ab3cv

Although the cost is high I recommend the use of Cool-Amp
_ ()
for use on small loops and RF ground busses. We used this
product at the shop on all the copper buss connections for the
platers to lower the resistance. It leaves a thin silver coating on
copper and brass. I have used it on PC boards as well as RF
buss. A little bit goes a long way. Anything that can be done to
reduce the skin resistance has a good pay off in the end.

Eric

At 2:41 pm ((PDT)) Mon Jun 18, 2007, christrask wrote:

If using a dipole, I would like to offer an alternative circuit, shown here:

Don't you need _very_ well matched JFETs
to ensure drain current balance in that circuit?

Could this be avoided by giving each JFET source
an independent primary and 330 ohm resistor,
then linking the "centre-tap" with the capacitor?

(Also giving extra degeneration to common-mode.)


Regards, LenW
--
From Yahoo! Groups Help: ... trim all the irrelevant quoted text
out of your message (as a courtesy to the other members of
the group to make the digest easier to read).

At 12:52 pm ((PDT)) Mon Jun 18, 2007, timo.nieminen wrote:

Referring to the schema by Chris Trask


I ask for help on the 1:3 transformer.

[Now a 1:2 transformer]

What kind of (Amidon) ferrite
ring core were suitable for the purpose? How many turns? Wound
interleaving or on opposite sides of the core?

This is like asking us how much string you will need
when you won't tell us the size of your parcel ;-)

However, assuming you want your transformer as broadband
as possible, this may help you to 'roll your own'

"A Second Look at Fabricating Impedance Transformers
for Receiving Antennas"
John Bryant with Bill Bowers and Nick Hall-Patch, VE7DXR
DXing.info, July 2003

[Supersedes:
"Fabricating Impedance Transformers for Receiving Antennas"
z_transformers.pdf or impedance_transformer_bryant.doc]

While this gives much more experiment and testing...

"Impedance Matching Transformers for Receiving Antennas
at Medium and Lower Shortwave Frequencies"
Bill Bowers, John Bryant and Nick Hall-Patch, VE7DXR
DXing.info June 29 2003, revised July 24 2003


Regards, LenW
--
Content of a follow-up post should exceed quoted content. (rfc1855)

At 10:37 am ((PDT)) Mon Jun 18, 2007, ewdawso_1 wrote:

Thanks for your input.

In another thread: Loop antenna with coil

It really does help make sense of things
to keep them in threads, especially for
anyone foolhardy enough to visit the
Messages archive expecting to find
something sensible there.

(It's there- it's the finding that's difficult ;-)

So: one topic per thread and
one thread per topic, please,
until the (regrettable) natural
subject drift kills it.

When I looked at those photos I at first
thought it was for transmitting, however the air capacitor kind of
threw that out for me.

I still think it's for transmitting, just
not much power or there'll be sparks.

That isn't a puny capacitor, but it's not
easy to estimate the airgap from the photo.

Since the loop antenna I am building (my
first) is for listening to am only it will be without the coil.

Yes, there doesn't seem to be a lot of purpose
to accurate receiver matching at MF or HF, unless
it's a crystal set and antenna power is all you have.

The common techniques are:
a) a coupling loop
- for a multi-turn loop, less turns,
often a single turn,
- for a single-turn loop, a smaller size
usually one-fifth the diameter; or
b)a high-impedance preamp.


Regards, LenW

self resonance is the problem above a particular diameter. you can actually
see this as you watch the capacitor needed for a given frequency as you
increase the size.

efficiency is the ratio of the skin effect impedence over the sum of the
skin effect plus free space impedence (which is pretty small.) note that the
efficiency doesn't take into account any joints which will reduce the
efficiency even further.

73

jim ab3cv

--- In loopantennas@..., DavidGriffin <davidgriffin@...>
wrote:

Also, what is the LoopCalc program comparing to when it states that
a
certain configuration is , e.g., 51% efficient. Compared to a standard
dipole?

Hi David,

Loop efficiency relates to resistive losses in the loop. If there
were no losses the efficiency would be 100%. Small loops have very
low resistance due to radiation, on the order of tenths of an ohm.
Compare this to 50 ohms for a dipole up a half wavelength. At such
low loop resistance, losses in the conductor out of which the loop is
constructed become significant. That is why small loops are made out
of pipe rather than wire in order to increased surface area and why
copper is preferred over aluminum, as copper has better conductivity
(lower resistance).

Dave WA6YSO

_ ()
Here is a site that describes the trap thing very well. I cannot find the QST
article that I was thinking of when I first posted to you. Good luck and
great fun.

73 Glenn, K4DTT



*** See what's free at .

K4DTT@... wrote:

Install a 160 meter trap at equal distance from the feed point (half way around the perimeter) of the loop.

that part I understand,
it's what the trap consists of
and how to calculate and make it that I'm asking about.
--
GB & 73's
KA5OAI
Sam Morgan

I've been playing with KI6GD's LoopCalc program for small transmitting
loops. And I have a couple of questions for those of you who have used
it. First, I notice that when the circumference of the loop exceeds
0.33 wavelength the programs says such length is too large for a small
transmitting loop. That same sentiment is voiced in W2BRI's faq at

Although, Brian indicates 25% as the max circumference rather than 33%.
Does anyone know why either of these sizes is considered max for a small
transmitting loop?
Also, what is the LoopCalc program comparing to when it states that a
certain configuration is , e.g., 51% efficient. Compared to a standard
dipole?
Thanks, Dave

Sam wrote:

I want to use a ~265 foot horizontal 80m closed loop, on 160m
what I need/want to do is
open the far side of the loop when it is operated on 160m
and close it for 80m and above
how do I do that?

How about a 1/4 wave (at 160 m) shorted stub of transmission line across the far end of the loop? A balun and a ~30 meter roll of coax could be packaged up pretty small, compared to the size of the loop.

In a message dated 6/18/2007 11:00:49 PM Eastern Standard Time,
davidgriffin@... writes:
Although, Brian indicates 25% as the max circumference rather than 33%.
Does anyone know why either of these sizes is considered max for a small
transmitting loop?
Also, what is the LoopCalc program comparing to when it states that a
certain configuration is , e.g., 51% efficient. Compared to a standard
dipole?
Thanks, Dave


Hi Dave,
Once the circumference of a small transmitting loop is much more than
about 30-33% of a wavelength the loop becomes self-resonant or actually
resonant below the operating frequency, so there is no way to tune the loop.
I think all antenna efficiency programs use a resonant dipole in free
space as the standard for antenna efficiency.
73 Todd WD4NGG



*** See what's free at .