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eHam Forums => Elmers => Topic started by: ZR1PJA on April 17, 2008, 01:18:47 PM



Title: Snake Antenna
Post by: ZR1PJA on April 17, 2008, 01:18:47 PM
A few years ago I found a web site that gave a detailed description of how to build a snake antenna using 75ohm co-axial cable for transmitting. I can't find this info anymore.
Please can someone help with the info on building this antenna.
Many thanks
Paul
Brgds


Title: Snake Antenna
Post by: WW5AA on April 17, 2008, 01:33:44 PM
There are several designs for the "Snake Antenna" I have a design that works well. My email address is good on QRZ. Send an email and I'll forward a diagram.

73 de Lindy


Title: Snake Antenna
Post by: VK1OD on April 17, 2008, 01:56:04 PM

If by snake antenna, you mean a wavelength of coax laid out in a straight line on the ground, with the far end shorted or even terminated...

If this works, it can only be by virtue of failure of the shield to be effective.

Sounds like you need to find coax that doesn't behave like good coax should.

Doesn't that sound some warnings?

Owen


Title: Snake Antenna
Post by: W8JI on April 17, 2008, 03:19:03 PM
Owen,

The snake antenna functions because the shield is not grounded and the load intentionally induces common mode on the shield.

In summary it is really nothing but a longwire laid on the ground. It just happens that the longwire is made from coax, which also doubles as a way to get the antenna into the shack.

Because the design intentionally induces common mode on the shield the shield can be 100% effective and the snake will still be a snake.

Look at a drawing and you willl quickly see how it REALLY works, not why people sometimes claim it works.

73 Tom


Title: Snake Antenna
Post by: VK1OD on April 17, 2008, 03:43:41 PM

Tom,

The diagrams that I have seen show the 'snake' terminated at the far end in a s/c or resistor. The preamp is connected between inner conductor and outer conductor, so if fed with differential mode voltage only. (Common mode current flows to the preamp ground terminal.)

In that case, with effective shield, there would be no differential signal.

Do you have a reference for the design you are discussing?

Owen


Title: Snake Antenna
Post by: VK1OD on April 17, 2008, 05:31:25 PM


Here is a pic that claims configuration A to be W1FB's original Snake Antenna: http://www.isp.ca/ve3nh/snake2.jpg

If the short at the far end is indeed a short between inner and outer conductor and not a small loop, and the preamp responds to the voltage between inner conductor and outer conductor (differential voltage), then:

The only way in which differential current will be excited within the coax is by through-braid leakage. No coax is perfect, no coax has zero through-braid leakage.

It seems to me that the antenna described as above depends on through-braid leakage, and for that reason, its performance would be very dependant on coax quality, poor coax would give more sensitivity than good coax.

The other interesting aspect of the antenna as described is at that preamp is located at a voltage minimum, obviously another 'feature'!

Mind you, leaky transmission line antennas are not new.

Owen


Title: Snake Antenna
Post by: W8JI on April 17, 2008, 06:21:25 PM
That diagram is incorrect.

One variation of the Snake antenna by DeMaw didn't work at all. Doug told me himself he never actually used that antenna, but he got the idea from a friend. Turned out that friend's antenna actually grounded the center conductor at the far end and floated the shield. Without that ground the antenna was dead. A simple omission.

If you look at the later ON4UN Handbook you will see a "working snake", not the non-working model on the VE3's website. ON4UN also explains why the originally published snake does not work.

The ON4UN Handbook shows a refined correct model of the Snake. It has a 1:4 transformer at the far end and a ground connection.

73 Tom


Title: Snake Antenna
Post by: N6AJR on April 17, 2008, 07:55:24 PM
if an antenna that cheap and that simple worked, would we not see  every one using them, instead of being something even google can not find??


Title: Snake Antenna
Post by: WA3SKN on April 18, 2008, 05:45:56 AM
Well, I don't know the "terminology" but I would not recommend any antenna that lies on the ground as a "first choice" antenna!  There is a reason people install them in the air!
But, as a "compromise antenna", it does not sound too expensive not to try!  You can always re-use the coax after the project.
73s.

-Mike.


Title: Snake Antenna
Post by: ZR1PJA on April 18, 2008, 07:35:08 AM
Thanks for all the responses. Lindy I have emailed you yesterday as per you email address and have not got a reply so wondering if you got my address. FYI it is pjallen@mweb.co.za

Many thanks
Paul


Title: Snake Antenna
Post by: K4JSR on April 18, 2008, 09:47:41 AM
Speaking of Snake Antennas---
Didja hear about the viper that was so destitute that he couldn't afford a pit to hiss in?



Title: Snake Antenna
Post by: K7UNZ on April 18, 2008, 10:02:42 AM
Paul, the "snake" is really ment to be a lower-noise receiving antenna.  Kinda like a poor man's Beverage.

I've tried it a few times at several locations around the world (40/80 meters) and like just about any antenna, sometimes it works-sometimes it doesn't.

73, Jim/k7unz


Title: Snake Antenna
Post by: K3ANG on April 18, 2008, 08:41:04 PM
N6AJR,
Queried google's exact word search on 'snake antenna.'
Lots of links, but this one looked most interesting.

http://www.navymars.org/south/newsletter14.pdf

go to the bottom of page 13, continues to 14.
From 2004.
I don't belong to Navy Mars, never used the antenna, YMMV, usual caveats.

73
de K3ANG



Title: Snake Antenna
Post by: W4VR on April 19, 2008, 01:20:16 PM
Don't believe everything you read.  I used a coaxial snake antenna back in the 70's and it was fine for 160-meter low-noise reception...but you may need a pre-amp for this antenna for it to work to your liking.  Mine was 100 feet long and terminated into a 50-ohm resistor.


Title: Snake Antenna
Post by: W8JI on April 19, 2008, 05:48:16 PM
If you connect a Snake Antenna like on the VE3 website and like DeMaw originally published, it will be stone cold dead even with the best preamp in the world.

Unless the shield is open circuited someplace it will not receive squat. It doesn't take an Albert Einstein to figure out why.

If you connect a Snake like the ON4UN Handbook shows, it will work very well.

73 Tom


Title: Snake Antenna
Post by: W4VR on April 20, 2008, 04:11:40 PM
I forgot to say that the shield was left floating at the radio end on my snake antenna I used back in the 70's.


Title: Snake Antenna
Post by: NV2A on April 24, 2008, 09:45:25 AM
Some of the replies I've read would indicate that some folks don't realize this antenna is used primarily on 160 and frequently with an outboard amp such as a polomar.  They are used for receive only.  It worked just fine for me years ago.

I was skeptical when I first heard about.  They had me going at the DX club meeting.  I thought they were tired of the snipe hunting and left handed screwdriver kind of gags.  Turns out they were right!


Title: RE: Snake Antenna
Post by: W0BKR on December 17, 2009, 09:21:29 AM
So, for property poor individuals like myself, what would be a good "snake" design length and construction tid bits. I too have seen lots of "info" but what is the real poop on this antenna and how to lay one out?

I have a FLAG antenna that does hear some stuff but it is really weak.  Could be the Sloper on my tower behind the FLAG (for a tie off support) may be affecting it's performance. I have no way to tell.  I  have a remote coax switch at the base of the tower and take the Sloper out of the line to the radio, but it is still attached to teh tower.  Could this be an issue?

Perhaps someone can share a bit of info to all these questions/concerns....