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Author Topic: A Sleeve Dipole Antenna for 2m  (Read 9237 times)
VU2NAN
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« on: August 21, 2011, 10:27:45 AM »

Here's the link.

http://nandustips.blogspot.com/2011/08/sleeve-dipole-antenna-for-2m.html

73

Nandu.
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W8JI
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« Reply #1 on: August 27, 2011, 01:40:32 AM »

The effectiveness of the sleeve in decoupling the coax shield is determined by the impedance at the open end of the sleeve where the connector is.


Because of that the sleeve diameter should be very large compared to the diameter of the coaxial cable shield. The insulation should be virtually all air, and three or four radials might still be needed on the coax shield just before it reaches the antenna.

See:

http://www.w8ji.com/sleeve_baluns.htm
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VU2NAN
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« Reply #2 on: August 27, 2011, 06:52:59 AM »

Hi OM Tom,

Thank you very much for all the info on sleeve baluns and the requirements for an effective sleeve dipole antenna.

73

Nandu.

P.S. It is confirmed that my small diameter sleeve is not very effective. The SWR is down to 1.5:1 from 3:1 after coiling the coax at the BNC (choke balun).
« Last Edit: August 27, 2011, 10:13:44 PM by VU2NAN » Logged
KA9ZIM
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« Reply #3 on: September 08, 2011, 02:57:28 PM »

I'm trying to tune a variation of this antenna.  I built an Off Center Fed 2M antenna using aluminum tape on a piece of 3/4" PVC pipe.  I have the SWR on 2M at 1.2:1 in the middle of the 2M band and I can go up to 155 MHZ before the SWR gets about 2:1.  Really nice broad band 2M antenna.  I'd like to use it on 440 also but the antenna seems to be optimum at 438 Mhz.  As I get up to 442 Mhz the SWR hits 2.5:1.  I was expecting this antenna to be usable on 440.  Any thought on why I'm fine on 2M but I seem to have missed the 440 Mhz band - the 440 FM band that is?

I am decoupling the RF with 4 turns of coax just below the antenna. 

Thanks!
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W8JI
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« Reply #4 on: September 08, 2011, 03:57:59 PM »

1.) You don't have a good RF choke at all, so the coax is part of the antenna.

2.) Even if you had a good choke, the third overtone of the antenna is never exactly 3x the fundamental. The spread depends on many things, including conductor size and the ratio of the lenegths on each half.
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WB6BYU
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« Reply #5 on: September 09, 2011, 07:01:05 AM »

Quote from: KA9ZIM
...I built an Off Center Fed 2M antenna ... 

..I was expecting this antenna to be usable on 440...

This does depend a lot on the design you are building.  A typical OCFD for 40m does NOT work well on the
third harmonic at 15m due to high SWR - in general they only work on even multiples of the design frequency.

There are some variants that you might be able to get to work, but I wouldn't count on it unless the antenna
is originally designed to do so.
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KB1LKR
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« Reply #6 on: September 10, 2011, 12:36:04 PM »

"I built an Off Center Fed 2M antenna using aluminum tape on a piece of 3/4" PVC pipe.  I have the SWR on 2M at 1.2:1 in the middle of the 2M band and I can go up to 155 MHZ before the SWR gets about 2:1.  Really nice broad band 2M antenna.  I'd like to use it on 440 also but the antenna seems to be optimum at 438 Mhz.  As I get up to 442 Mhz the SWR hits 2.5..." KA9ZIM

What happens if you trim it a bit 'til you're at 2:1 at perhaps 150-152 MHz? Does that drive the 70cm min. SWR up into the mid 440's? Where's the lower 2:1 point now, i.e. what's its bandwidth on 2m now; will it tolerate trimming?

Why OCF vs center fed for 2m anyway?
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KB1LKR
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« Reply #7 on: September 10, 2011, 12:44:49 PM »

"... the sleeve diameter should be very large compared to the diameter of the coaxial cable shield..."  -- W8JI

What's "very large"? Ten times the coax shield dia., e.g. about 2"-2.5" for RG-58 or RG-8X sized coax?

 
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W8JI
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« Reply #8 on: September 10, 2011, 01:53:14 PM »

"... the sleeve diameter should be very large compared to the diameter of the coaxial cable shield..."  -- W8JI

What's "very large"? Ten times the coax shield dia., e.g. about 2"-2.5" for RG-58 or RG-8X sized coax?

 

The larger the better as a general rule, although I'm sure there is an upper limit.

There was a cone decoupled 2 meter antenna that worked pretty well, the isopole.
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WB6BYU
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« Reply #9 on: September 10, 2011, 05:18:22 PM »

A couple inches is probably adequate.  You certainly want to make sure that most of the
dielectric is air rather than the outer insulation on the coax.

I've got a couple Isopoles and they appear to work well.  The top cone is the bottom of the
radiator and the one below it is for decoupling.  They actually fit over the supporting mast,
which the coax runs down inside.  I'd guess the end diameter is about 9" or so on 2m.  They
use a stiff foam insulator for mechanical support about in the middle of each cone to avoid
anything at the bottom end, which is a high impedance.
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W8JI
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« Reply #10 on: September 11, 2011, 09:13:57 AM »

Actually a few inches is not enough for a single sleeve to decouple the coax all by itself. It really has to flare out a considerable amount, and even then will still have significant shield currents.

The Isopole was a big step in the right direction. Good thinking on their parts, but usually it will take a combo of sleeve and radials or an additional choke.
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