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Author Topic: Isotron antennas: For real?  (Read 5417 times)
WB6BYU
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Posts: 17277




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« Reply #15 on: January 01, 2018, 06:11:44 PM »

Quote from: AC2EU

How do these "zombie posts" happen?



Somebody is reading old posts - perhaps they did a search or followed a link, rather than starting
at the most current page and working backwards.  They find something they want to comment on,
and add a post.  Simple as that.  Old threads don't get locked.

The software does query whether you really want to reply to a post that old - I think the criteria
is around 6 months or so - but, if you say "yes", then it posts your comments, just as it would
for a more recent topic.

And, yes, people do read old threads, especially when they do an internet search on a topic and
find some useful comments.  This one was from 2003, which is a while back for eHam, but I've
gotten email queries on posts that were further back than that.


It just goes to show that some topics never go out of date...



And the Isotron hasn't changed in the meantime:  if you remove the antenna, clamp the coax
shield to the mounting post, and feed the bottom end of the coax shield as an end fed wire, it
may work about the same.  Or better...
« Last Edit: January 01, 2018, 06:14:17 PM by WB6BYU » Logged
K0UA
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Posts: 1625




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« Reply #16 on: January 01, 2018, 08:58:28 PM »

it may work about the same.  Or better...

I would bet on the better....
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AC2EU
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Posts: 1240


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« Reply #17 on: January 02, 2018, 07:05:14 AM »

"ouch" on both counts!   Grin

I guess you can think of it as a fancy Tesla coil which is also known to be a poor EM radiator, but has a powerful near field.
However, to Tesla's defense, the original concept was to create an ion path between towers at 30, 000 feet using balloons.
The Isotron does not mention any such requirement, thus doomed to be a cloud warmer like the compromise concept of Wardenclyfe.
I read that even with hundreds of thousands of watts, it was only effective at less than 200 miles.

Rule of thumb:
If the tuning reactance is large compared to the radiation resistance, you have low efficiency.
The isotron also has the skin resistance of the wire that competes with the radiation resistance for energy.

As mentioned earlier, It is possible to make contacts with a 100W light bulb too!
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WB6BYU
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Posts: 17277




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« Reply #18 on: January 02, 2018, 07:48:52 AM »

There are a lot of small antenna designs, both commercial and homebrew,
where the coax shield is responsible for more radiation than the "antenna"
itself.  Some may warn you not to use a feedline choke at the antenna,
and some (the TAKtenna comes to mind) provide tuning instructions that
maximize coax radiation even though the antenna may appear balanced.

These might be useful in some situations, but users should be told that is
the case so they can plan their installations accordingly.  I was checking
out a 40m Isotron with an SWR analyzer and couldn't find any sign of
HF resonance using a short patch cable.  After checking every joint for
a bad connection (there aren't that many) the problem became obvious -
it only worked with a long cable to act as the radiator.  If that cable can
be run along the peak of the roof rather than laying on the ground, the
"antenna" will work much better.
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AC2EU
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« Reply #19 on: January 02, 2018, 01:41:14 PM »

There are a lot of small antenna designs, both commercial and homebrew,
where the coax shield is responsible for more radiation than the "antenna"
itself.  Some may warn you not to use a feedline choke at the antenna,
and some (the TAKtenna comes to mind) provide tuning instructions that
maximize coax radiation even though the antenna may appear balanced.

These might be useful in some situations, but users should be told that is
the case so they can plan their installations accordingly.  I was checking
out a 40m Isotron with an SWR analyzer and couldn't find any sign of
HF resonance using a short patch cable.  After checking every joint for
a bad connection (there aren't that many) the problem became obvious -
it only worked with a long cable to act as the radiator.  If that cable can
be run along the peak of the roof rather than laying on the ground, the
"antenna" will work much better.

Are those Isotrons tune-able? The cap is probably very touchy to tune like most loops.
I read in one of the reviews that a Ham had a lot of trouble dialing the match in.
I think he ended up taking a turn off the coil too!
The company had to walk him through the mods.

Seems like the thing is more trouble than it's worth. I could make one like it for a whole lot less money! i would probably work better too- that is, as good as something like that can work...
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KX4OM
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Posts: 217




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« Reply #20 on: January 02, 2018, 04:09:41 PM »

If you really want to twist your mind around something, read the 2010 QEX article by W5ZN and N4HY, "Design, Construction and Evaluation of the Eight Circle Vertical Array for Low Band Receiving." It is based on previous work by O4UN, W8JI and others. In horizontal acreage, the thing is not small, but the radiating vertical elements and capacity hat wires are only 25 feet in length (for 160 and 80 meters!) Working my way through the 16-page article and its 4nec2 modeling and construction and testing details forced me to think about the importance of height, wavelength, ground, and noise differently from the standard dipole vs. vertical HF antenna theory and arguments about "compromise" antennas.

Similarly, the very small HF broadband voltage probe antenna based on PA0RDT's design at The University of Twente SDR operation tweaks the mind, but the thing works, and it is not apparently just due to the feedline, because the feedline passes through the grounded metal roof of the building to the receiving equipment (an SDR) just below the mounting point of the antenna.

Info on the homebrew SDR build over several years:

http://www.pa3fwm.nl/projects/sdr/

"Such an antenna only works well with a good grounding; ours is on top of a 20m high building, the upper part of which is all metal." Quote from Pieter, PA3FWM from the page below, which is also the WebSDR page:

http://websdr.ewi.utwente.nl:8901/

Ted, KX4OM
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WB6BYU
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Posts: 17277




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« Reply #21 on: January 02, 2018, 09:29:22 PM »

Quote from: AC2EU

Are those Isotrons tune-able? The cap is probably very touchy to tune like most loops...




Yes, they have some sort of tuning arrangement that adjusts the capacitance between the
top and bottom plates.   And, yes, they can be particularly difficult to tune, especially if
you have to take it down and put it back up each time, since the way the excess coax is
coiled up on the ground will affect the tuning:  you can take it down and put it back up and
have the resonant frequency change without making any other adjustments to the antenna.
That really makes it interesting to get it tuned up!  In my case I mounted a 40m Isotron on a
pole and searched the whole HF range looking for a dip, and never found one.

I never got mine to tune up at all.  Once I had verified that all the connections really were
good, and the reason that I couldn't get a low SWR was because I was using a very short
jumper to the SWR analyzer, I gave it away to another ham who wanted to experiment with it.
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WA3SKN
Member

Posts: 6558




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« Reply #22 on: January 05, 2018, 08:16:00 AM »

The Isotron is nothing more than a "tank circuit", a variable coil/capacitor in parallel, and made to look like 50 ohms to the transmitter.
It is small and tries to radiate.  It is not a good antenna, but "some times you just have to work with what you got".  It will work better than a dummy load.
It is tunable, and yes... it has a high Q.
73s.

-Mike.
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N3OX
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Posts: 8911


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« Reply #23 on: January 13, 2018, 12:29:57 PM »

And, yes, they can be particularly difficult to tune, especially if
you have to take it down and put it back up each time, since the way the excess coax is
coiled up on the ground will affect the tuning

Yeah, this was challenging to tune:
http://n3ox.net/files/tunerant/tuner_ant.jpg
http://n3ox.net/files/tunerant/tuner_connex.jpg

So I eventually figured it'd be easier to put the tuner at the bottom end  Grin

It did work though.

(P.S. disclaimer: this is probably terrible for the metering circuitry)
« Last Edit: January 13, 2018, 12:32:06 PM by N3OX » Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KD8IIC
Member

Posts: 700




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« Reply #24 on: January 13, 2018, 06:27:59 PM »

  If they indeed work so well, why have I yet to EVER have someone tell me that they are using one during a fone or
  CW QSO since 2008??  Wire antenna = money well spent.
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KM1H
Member

Posts: 3083




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« Reply #25 on: January 14, 2018, 11:53:20 AM »

These might be useful in some situations, but users should be told that is
the case so they can plan their installations accordingly.  I was checking
out a 40m Isotron with an SWR analyzer and couldn't find any sign of
HF resonance using a short patch cable.  After checking every joint for
a bad connection (there aren't that many) the problem became obvious -
it only worked with a long cable to act as the radiator.  If that cable can
be run along the peak of the roof rather than laying on the ground, the
"antenna" will work much better.
Logged
KM1H
Member

Posts: 3083




Ignore
« Reply #26 on: January 14, 2018, 12:22:36 PM »

Quote
These might be useful in some situations, but users should be told that is
the case so they can plan their installations accordingly.  I was checking
out a 40m Isotron with an SWR analyzer and couldn't find any sign of
HF resonance using a short patch cable.  After checking every joint for
a bad connection (there aren't that many) the problem became obvious -
it only worked with a long cable to act as the radiator.  If that cable can
be run along the peak of the roof rather than laying on the ground, the
"antenna" will work much better.

Switch in sections of coax to add more bands. Grin

Around 1990 and after I moved here a couple of us 160M DXers were gabbing on a semi private 222 mHz simplex frequency using old Clegg and Midland xtal controlled 10W FM rigs; the Clegg was a rebadged Midland.

The subject of lightbulb antennas came up but none had a readily available setup. SO, the Heath Cantenna dummy load was used by 3 of us. I was at one end of this invisible line and about 31 and 25 miles from the others who were in MA and a straight line from here. With 100W on 160 we held a ~10-15 minute chat with signals barely out of the rig noise but a full Q5 copy.

I was in the basement but on top of a 650' hill and the others were on the first floor on flat ground. Random 10-20' lengths of RG-58 were used and no ferrite chokes. No amount of looking for loose connections had any effect.

Try it sometime and go for a world record.

Carl
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K0UA
Member

Posts: 1625




Ignore
« Reply #27 on: January 14, 2018, 01:09:32 PM »

Quote
These might be useful in some situations, but users should be told that is
the case so they can plan their installations accordingly.  I was checking
out a 40m Isotron with an SWR analyzer and couldn't find any sign of
HF resonance using a short patch cable.  After checking every joint for
a bad connection (there aren't that many) the problem became obvious -
it only worked with a long cable to act as the radiator.  If that cable can
be run along the peak of the roof rather than laying on the ground, the
"antenna" will work much better.

Switch in sections of coax to add more bands. Grin


Add a coax T to that dummy load, and on the open port, add some wire.  Throw it up in a tree if you can.  Now you have a MAXX-COMM auto antenna tuner... TAK-tenna, or CHA250B etc etc.  Works about as well.  Good SWR on all frequencies.  Smiley  "tunes instantly" 
Around 1990 and after I moved here a couple of us 160M DXers were gabbing on a semi private 222 mHz simplex frequency using old Clegg and Midland xtal controlled 10W FM rigs; the Clegg was a rebadged Midland.

The subject of lightbulb antennas came up but none had a readily available setup. SO, the Heath Cantenna dummy load was used by 3 of us. I was at one end of this invisible line and about 31 and 25 miles from the others who were in MA and a straight line from here. With 100W on 160 we held a ~10-15 minute chat with signals barely out of the rig noise but a full Q5 copy.

I was in the basement but on top of a 650' hill and the others were on the first floor on flat ground. Random 10-20' lengths of RG-58 were used and no ferrite chokes. No amount of looking for loose connections had any effect.

Try it sometime and go for a world record.

Carl

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KV7W
Member

Posts: 157




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« Reply #28 on: January 15, 2018, 08:19:58 AM »

2003 post.

-Mike.

How do these "zombie posts" happen?

This is the internet, no such thing as a "zombie" post. Even though we tend to treat the internet like a physical bookcase that can fill up or a public meeting where redundant topics and redundant sidebar topics should be avoided; history is fluid here and time isn't all that relative. There's nothing wrong with responding to a 100 year old post. In a distributed network where millions of terabytes of data get moved around every second - ideas are timeless. 
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