wrote:
I'm thinking it would be fun to get some wire, a capacitor and a jumbo
sized ferrite rod to make a good ferrite loop antenna. Does anyone know
where you can buy oversized ferrite rods (say, ten inches or more in
length) without spending an arm and a leg?

Steve
++++++++++++++++++
For some ideas that work look up:
Burhans, RW; "VLF active antennas," Radio Electronics, pp. 63-66,
February 1983.(pdf)
Burhans, RW; "VLF active antennas-part 2," Radio Electronics, pp.
65-72, March 1983.(pdf)
at http://www.rsl.ku.edu/~eecs501/active_antennas/

His unusual "split/counter" wound ferrite loop works very well.
Smaller Ferrite rods will work at higher freqs.

Email listed is no longer valid.
Terry

The Burhans papers are on vertical whips. They are really noisy for HF.
But the list of papers is impressive.


wrote:
wrote:
I'm thinking it would be fun to get some wire, a capacitor and a jumbo
sized ferrite rod to make a good ferrite loop antenna. Does anyone know
where you can buy oversized ferrite rods (say, ten inches or more in
length) without spending an arm and a leg?

Steve
++++++++++++++++++
For some ideas that work look up:
Burhans, RW; "VLF active antennas," Radio Electronics, pp. 63-66,
February 1983.(pdf)
Burhans, RW; "VLF active antennas-part 2," Radio Electronics, pp.
65-72, March 1983.(pdf)
at http://www.rsl.ku.edu/~eecs501/active_antennas/

His unusual "split/counter" wound ferrite loop works very well.
Smaller Ferrite rods will work at higher freqs.

Email listed is no longer valid.
Terry

wrote:
The Burhans papers are on vertical whips. They are really noisy for HF.
But the list of papers is impressive.

Sorry, somewhere I have a link to different style loop that he
desinged.
Will try to find and post it today.

Terry

wrote:
The Burhans papers are on vertical whips. They are really noisy for HF.
But the list of papers is impressive.
--------------------------
I can't find the link to a pdf of the entire article, so try:
http://user.netonecom.net/~swordman/Radio/re-loop-article.htm

Terry

This is a good article. I've often wondered about that ferrite with the
two windings on it.
http://user.netonecom.net/~swordman/...Loops_Fig9.jpg
That is how my RDF antenna is designed. If you look at many of the old
boating RDFs, they use a stereo phone plug/jack as the interface, i.e.
two wire ends plus ground. The ferrite sits in a copper shield.

Untuned loops really make sense if your radio is decent. That is one
reason I like the wellbrook. But this paper has given me a few ideas on
how to use a RDF loop. Incidentally, the ferrite rods from these RDFs
show up occasionally at flea markets. The radio has crapped out, but
someone wisely saved the ferrite. I saw one on sale for $2 at the last
Livermore swap meet.



wrote:
wrote:
The Burhans papers are on vertical whips. They are really noisy for HF.
But the list of papers is impressive.
--------------------------
I can't find the link to a pdf of the entire article, so try:
http://user.netonecom.net/~swordman/Radio/re-loop-article.htm

Terry

wrote:
This is a good article. I've often wondered about that ferrite with the
two windings on it.
http://user.netonecom.net/~swordman/...Loops_Fig9.jpg
That is how my RDF antenna is designed. If you look at many of the old
boating RDFs, they use a stereo phone plug/jack as the interface, i.e.
two wire ends plus ground. The ferrite sits in a copper shield.

Untuned loops really make sense if your radio is decent. That is one
reason I like the wellbrook. But this paper has given me a few ideas on
how to use a RDF loop. Incidentally, the ferrite rods from these RDFs
show up occasionally at flea markets. The radio has crapped out, but
someone wisely saved the ferrite. I saw one on sale for $2 at the last
Livermore swap meet.
++++++++++++++++++++++++++
By using Litz wire and taking some care it is "easy" to get a ~10" X
3/4"
rod to work up to around 10MHz. Above that it goes downhill "real
fast".

I ran into this winding on a dead RCA RDF a friend gave me. I rembered
seeing the article back in Radio Electronics but had completly
forgotten.

Burhans had some designs that were light years a head of their time!

I wish I had payed more attention when they first came out. It was a
royal
PITA to get photocpies from the local university's enginering library
microfilm
editions of Radio Electronics.

Terry

Is the skin effect significant because you are trying to get lots of
windings which implies fine wire? Also, what about spacing between the
wire (i.e. interwinding capacitance)?

One of the papers mentioned the higher permeability cores had lower
bandwidth.

I've been fiddling with multiple turns with the Wellbrook ALA100. I've
got so much signal strength on local BCB that I can hear an internal
relay switching in an attenuating pad.

wrote:
wrote:
This is a good article. I've often wondered about that ferrite with the
two windings on it.
http://user.netonecom.net/~swordman/...Loops_Fig9.jpg
That is how my RDF antenna is designed. If you look at many of the old
boating RDFs, they use a stereo phone plug/jack as the interface, i.e.
two wire ends plus ground. The ferrite sits in a copper shield.

Untuned loops really make sense if your radio is decent. That is one
reason I like the wellbrook. But this paper has given me a few ideas on
how to use a RDF loop. Incidentally, the ferrite rods from these RDFs
show up occasionally at flea markets. The radio has crapped out, but
someone wisely saved the ferrite. I saw one on sale for $2 at the last
Livermore swap meet.
++++++++++++++++++++++++++
By using Litz wire and taking some care it is "easy" to get a ~10" X
3/4"
rod to work up to around 10MHz. Above that it goes downhill "real
fast".

I ran into this winding on a dead RCA RDF a friend gave me. I rembered
seeing the article back in Radio Electronics but had completly
forgotten.

Burhans had some designs that were light years a head of their time!

I wish I had payed more attention when they first came out. It was a
royal
PITA to get photocpies from the local university's enginering library
microfilm
editions of Radio Electronics.

Terry

wrote:
The Burhans papers are on vertical whips. They are really noisy for HF.
But the list of papers is impressive.
++++++++++++++++++++++++++++++
Properly designed (and constructed) active antennas with a small (3'
or less) vertical
element can work very well. And in the right location will give a good
100' wire antenna
stiff competition. The trick is getting active devices good enough to
give acceptable
IMD performance. The JFETs Burhans are amazing and are still amoung the
best
easy to get devices avaliable.

I like the AmRad LF antenna as it still has enough gain to be very
usefull up to above
10M. The North Country Radio Active antenna is a lot chepaer and comes
as a nearly complete kit.
For construction details see:
http://www.northcountryradio.com/Published/actant_0896/page1.htm
For the ~45$ it represents a pretty good deal for anyone who needs an
active antenna.

Yodar in the shortwave antenna group mentioned they have changed the
FET and it
appears to be more resitant to IM.

Whatever you do avoid the McKay Dymek 100D! The 100E is now being sold,
but the
100D I tested was less then usefull. It belonged to a friend and was
NIB, but had set
around for a few years. The 100E could be decent or even great, but I
would as soon
as use a 50 Ohm dummy load as use the 100D.

Terry

Aren't the whips just E-field sensitive? That is why they are noisy.
I'm not sure a better FET can get around a fundamental problem like
that.
wrote:
wrote:
The Burhans papers are on vertical whips. They are really noisy for HF.
But the list of papers is impressive.
++++++++++++++++++++++++++++++
Properly designed (and constructed) active antennas with a small (3'
or less) vertical
element can work very well. And in the right location will give a good
100' wire antenna
stiff competition. The trick is getting active devices good enough to
give acceptable
IMD performance. The JFETs Burhans are amazing and are still amoung the
best
easy to get devices avaliable.

I like the AmRad LF antenna as it still has enough gain to be very
usefull up to above
10M. The North Country Radio Active antenna is a lot chepaer and comes
as a nearly complete kit.
For construction details see:
http://www.northcountryradio.com/Published/actant_0896/page1.htm
For the ~45$ it represents a pretty good deal for anyone who needs an
active antenna.

Yodar in the shortwave antenna group mentioned they have changed the
FET and it
appears to be more resitant to IM.

Whatever you do avoid the McKay Dymek 100D! The 100E is now being sold,
but the
100D I tested was less then usefull. It belonged to a friend and was
NIB, but had set
around for a few years. The 100E could be decent or even great, but I
would as soon
as use a 50 Ohm dummy load as use the 100D.

Terry

wrote:
Aren't the whips just E-field sensitive? That is why they are noisy.
I'm not sure a better FET can get around a fundamental problem like
that.
+++++++++++++++++++
It all depends...in most cases a horizontal is the better choice. But
in some
locations you can use an short, active antenna as a better choice
because
you can move the smaller antenna around to sniff out the least noisy
spot.
An example, here at work we have gobs of class A computer devices.
Servers, process controllers etc. RF hell so to speak.

We need an backup time retrieval system. We use GPS, but during a big
ice
storm 3 years ago the 1" of ice killed it. Had we lost our T1 we would
have
been in deep trouble. We have some experiments that have to be time
stamped.
So I pitched the idea of a WWVB and or WWV backup system. We tried a
10'
through 60' wire antenna at several locations on the roof. But there
was just too much
noise. Then I brought in my AmRad antenna and found several locations
that were
quite enough for a WWV decoder to lock to the ~BCD signal WWV sends.
WWVB
was a washout as we have a strong, out of house, spur smack dab on top
of 60KHz.
I did have to take "extreme" precautions to keep RF from creeping up
the braid and
getting into the antenna by that patch. Lots of feritte torroids on the
coax. We went
with a homebrew version very similar to one shown in Radio Electronics
with a hefty
FET. No IM, yet. I have a "feed" from the receiver brought out at my
bench so I can
keep an ear on the audio. I also have a dedicated PC to look at the
"time" as shown
by GPS, NIST via the web, and WWV.

I have several friends who live in restrictive residential areas that
do not allow
out side antennas. In such situations it is easier to hide an short
E-field active
antenna then any sort of "long" wire.

For ~99% of SWL applications I suspect a good wire antena is the better
choice.
A wire antenna is always my first choice. For the same ~50$ as the
North Country antenna, the least expensive and good antenna that I know
of, I could errect a killer
wire antenna. But active antennas can do a good job and in the right
location can do
a extremely good job. The biggest problem from my personal experience
isn't normal QRN/QRM noise like static, but MW IM. All active antennas
suffer from this to some degree! The closer one is to a MW station,
especially 2 or more stations, the greater
this threat is. I suspect, but have no experience, that "in band", or
SW stations, nearby would cause even more IM issues.


Terry