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Author Topic: compromise antenna for 40 or 40'/20  (Read 664 times)

K2JF

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compromise antenna for 40 or 40'/20
« on: October 06, 2021, 01:35:44 PM »

These workers are up on the roof of my NYC apartment building doing renovation work. Pain in the butt because they took down my dipoles. Is there a compromise antenna I can stick up there that can be mounted on the outer perimeter of the roof to get me on the air while these pains finish their work and get the heck down out of there?
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WB6BYU

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Re: compromise antenna for 40 or 40'/20
« Reply #1 on: October 06, 2021, 06:26:26 PM »

You mean like a mobile whip?

K2JF

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Re: compromise antenna for 40 or 40'/20
« Reply #2 on: October 07, 2021, 03:01:51 AM »

that's a possibility but I'm looking for something that might work a little better. I'm looking at the Isotron in spite of it's share of nasty reviews!
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N6XJP

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Re: compromise antenna for 40 or 40'/20
« Reply #3 on: October 07, 2021, 05:27:56 AM »

Do you have a balcony?
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K2JF

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Re: compromise antenna for 40 or 40'/20
« Reply #4 on: October 07, 2021, 09:40:07 AM »

I have a railing that runs along the perimeter of the roof. am thinking of an end fed wire antenna up there
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WB6BYU

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Re: compromise antenna for 40 or 40'/20
« Reply #5 on: October 07, 2021, 11:46:49 AM »

Quote from: K2JF

...I'm looking at the Isotron in spite of it's share of nasty reviews!




The Isotron can give good results if mounted on a long enough
feedline or mast, since that is what does the radiating, rather
than the antenna contraption itself.

K2JF

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Re: compromise antenna for 40 or 40'/20
« Reply #6 on: October 07, 2021, 11:50:59 AM »

I'll probably stay away from Isotron and will find a way to put up one of the end fed antennas I have sitting around.
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K0UA

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Re: compromise antenna for 40 or 40'/20
« Reply #7 on: October 07, 2021, 03:01:32 PM »

Quote from: K2JF

...I'm looking at the Isotron in spite of it's share of nasty reviews!




The Isotron can give good results if mounted on a long enough
feedline or mast, since that is what does the radiating, rather
than the antenna contraption itself.

Hm... you have just given me food for thought for something I can sell. Perhaps a nice dummy load with some good leaky Chinese coax. probably work about as well as some antennas reviewed on this forum. It would be a good "multiband from 1.8 to 54 Mhz antenna with a very low SWR and "good bandwidth". And it would have a good "gain number" albeit with a minus sign in front of it. I am thinking this could sell.  :)
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73  James K0UA

KM1H

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Re: compromise antenna for 40 or 40'/20
« Reply #8 on: October 07, 2021, 05:16:27 PM »

Give the workers a couple of cases of beer and have them install antennas.

Carl
An EX NYC, Brooklyn. Nassau County op from 1955 forward
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N6YWU

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Re: compromise antenna for 40 or 40'/20
« Reply #9 on: December 02, 2021, 12:29:52 PM »

The Isotron can give good results if mounted on a long enough
feedline or mast, since that is what does the radiating, rather
than the antenna contraption itself.

I wonder if anyone has tested this hypothesis by transmitting without any feedline (or conductive mast) at all ?

(using something like a battery powered Tiny Tin transmitter screwed directly to the antenna feedpoint; maybe Bluetooth keyed, or keyed via a colocated Arduino.)
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WB6BYU

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Re: compromise antenna for 40 or 40'/20
« Reply #10 on: December 02, 2021, 02:15:04 PM »

Quote from: N6YWU

I wonder if anyone has tested this hypothesis by transmitting without any feedline (or conductive mast) at all ?



Yes, in various ways.

I spent over a day trying to get a 40m version to
show any sort of dip on an SWR analyzer,
Using something like an 18” jumper cable.  Double
and triple checked every connection.

I gave it away to someone else who wanted to try
it, and they got the same results.

Others have reported the SWR varying as the coax
is rearranged, without any other changes to the
antenna itself.  And adding a choke / current balun
at the antenna drove the SWR off scale, with no dip
anywhere close to the design frequency.

In fact, the manufacturer had specifically said NOT
to use a choke / balun with it.

But it isn’t unique in that regard - many of the claims
about small antennas have been found to rely on
radiation from the coax rather than from the antenna
itself.  If you can mount it to a tower with a long enough
feedline, they may, in fact, be better than no antenna
at all, and get you on the air when it isn’t convenient
to put up a more effective antenna.

KH6AQ

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Re: compromise antenna for 40 or 40'/20
« Reply #11 on: January 11, 2022, 04:24:32 PM »

The Isotron can give good results if mounted on a long enough
feedline or mast, since that is what does the radiating, rather
than the antenna contraption itself.

I wonder if anyone has tested this hypothesis by transmitting without any feedline (or conductive mast) at all ?

(using something like a battery powered Tiny Tin transmitter screwed directly to the antenna feedpoint; maybe Bluetooth keyed, or keyed via a colocated Arduino.)

Yes, see my article THE ISOTRON REVEALED - PART 2

https://studyres.com/doc/7899181/the-isotron-revealed---part-2-by-dave-cuthbert

Part 1 is unavailable and I do not have a copy. AntenneX magazine closed and the first part of the article may be lost forever. In part 1 an actual 40 meter isotron was driven by an MFJ-259 Antenna Analyzer directly at the Isotron. It was attached by a male-to-male adapter and the isotron was tuned to resonance. The isotron and analyzer were then pulled up a 25' wooden mast for field strength testing to compare against a NEC-2 model.
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KH6AQ

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Re: compromise antenna for 40 or 40'/20
« Reply #12 on: January 12, 2022, 05:30:54 AM »

I found my copy of the article, The Isotron Revealed (part 1). I cannot post it because AntenneX magazine owns the copyright, however I have posted two pics of the test setup and field strength measurements.

Photo of the 40 meter Isotron with MFJ-259 connected directly:               https://imgur.com/cNTlLwq

Photo of the 40 meter Isotron hoised to a height of 20' on wooden mast:  https://imgur.com/8EwJl7g


The results with and without the 20' counterpoise:

20' counterpoise 48 dBuV
No counterpoise 37 dBuV
Difference          11 dB

The measured field strength difference is 11 dB while the simulated difference is 16 dB.
« Last Edit: January 12, 2022, 05:49:58 AM by KH6AQ »
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