Author Topic: Antennas  (Read 72553 times)

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kew

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Re: Antennas
« Reply #20 on: Mar 22, 07, 08:47:05 PM »
This site has a calculator for determining the length for each section of a J-pole at any freq.

http://www.hamuniverse.com/jpole.html

Offline RobertW

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Re: Antennas
« Reply #21 on: Mar 22, 07, 08:47:25 PM »
In the March '07 issue of the ARRL QST magazine they revisited making a portable roll-up J-pole antenna for 2 meter/70 cm.

With Clint making it very clear that a lot of HAMs don't know what they are talking about, does this guy know what he's talking about in building a J-Pole Antenna?:

http://www.alpharubicon.com/elect/jpolejaden.htm

It sounds pretty easy!

clint

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Re: Antennas
« Reply #22 on: Mar 22, 07, 09:14:04 PM »
I can tell you from expereince that there are many j-pole designs out there, and most of them don't work. One friend of mine WD6DZS compiled an compendeum of j-pole designs and in trying to build them at a workshop not one of them worked! The one I use does not require a balun loop and that's the first time I ever saw one called for.

By definition/design a J-pole is a 1/2 wave over a 1/4 wave measured from the feed point. These demensions look a little off to me but I have not checked them closely.
I don't like the designs that use a mounted SO-239 so this would not be one I would choose. But that's just a personal choice not a design flaw.

kew

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Re: Antennas
« Reply #23 on: Mar 22, 07, 09:32:02 PM »
This one isn't backpack portable but if it works it might be good for a near invisible antenna for RVs etc.

http://ka1fsb.home.att.net/jpole.html

clint

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Re: Antennas
« Reply #24 on: Mar 22, 07, 10:22:12 PM »
That KA1FSB site is one of the best J-Pole documents I've ever seen!

kew

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Re: Antennas
« Reply #25 on: Mar 23, 07, 05:56:50 AM »
Clint - The magnetic base antenna you showed us in class with the small diameter coax. What RG # is the coax and where did you get it?

Offline Wrightwood

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Re: Antennas
« Reply #26 on: Mar 23, 07, 06:52:02 AM »
Amyone have experience with limited space Slinky Dipoles and inverted Vee Antennas ?

http://www.kbzeroxa.com/the_slinky_ham_radio_dipolei.htm

Offline Toolman

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Re: Antennas
« Reply #27 on: Mar 23, 07, 02:24:55 PM »
Clint - The magnetic base antenna you showed us in class with the small diameter coax. What RG # is the coax and where did you get it?

Ken....if your looking for small coax for car installation or short runs, I personally do NOT use the RG-58 that radio shack sells. I prefer to use RG8-X for mobile installs or runs 50' or less. It has a higher gauge center conductor, insulation properties, and a higher coefficient velocity factor rating. Although the retaining ring for a Pl-259 fitting is available to fit the diameter, .242 = RG8-X vrs. .195= RG-58 you can still use the standard retaining piece by drilling it out. Or simply order the correct fittings when you order the coax. The RG-8X can maintain a tight radius for mobile installations.

Offline Toolman

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Re: Antennas
« Reply #28 on: Mar 23, 07, 02:28:01 PM »
Amyone have experience with limited space Slinky Dipoles and inverted Vee Antennas ?

http://www.kbzeroxa.com/the_slinky_ham_radio_dipolei.htm

Yep...built a lot of inverted vee center fed dipoles. As cheap as baluns are I just buy them. A Van Gordon is less than 20 bucks, but a high wattage balun will run about 75.
Those are what I was talking about in the beginning of the antenna post.

kew

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Re: Antennas
« Reply #29 on: Mar 23, 07, 04:53:35 PM »
Clint - The magnetic base antenna you showed us in class with the small diameter coax. What RG # is the coax and where did you get it?

Ken....if your looking for small coax for car installation or short runs, I personally do NOT use the RG-58 that radio shack sells. I prefer to use RG8-X for mobile installs or runs 50' or less. It has a higher gauge center conductor, insulation properties, and a higher coefficient velocity factor rating. Although the retaining ring for a Pl-259 fitting is available to fit the diameter, .242 = RG8-X vrs. .195= RG-58 you can still use the standard retaining piece by drilling it out. Or simply order the correct fittings when you order the coax. The RG-8X can maintain a tight radius for mobile installations.

At our Ham class, Clint had a mobile antenna with a magnetic base and a coax about 1/16" diameter and very flexible.
 

clint

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Re: Antennas
« Reply #30 on: Mar 23, 07, 11:58:18 PM »
Ken that was RG-174. It's horrible stuff and has almost no right to exist. Plus you can't install connectors on it without a special tool. It comes on those MFJ micro mag mounts and they are only suitable for emergencies!Mike's suggestion of RG-8X is right on for mobiles. What Larsen supplies on their NMOs is fine too.

kew

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Re: Antennas
« Reply #31 on: Mar 24, 07, 12:01:16 AM »
Thanks you, both. :)

Offline Toolman

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Re: Antennas
« Reply #32 on: Mar 24, 07, 02:52:35 AM »
RG-174. It's horrible stuff and has almost no right to exist.

 Well yes you have a good point there for our frq. usage. But it's actually used a lot at the upper frq. ranges in the 3gig and up range. It's used on a lot of spacecraft for telemetry and our RFI thruster EMI testing scientist at work in the anachoic chamber uses it exclusively. It's a pain to put sma connectors on it and "N" connectors.....I know...I make up his cables. :'(

 It has no use in our world of frq's that's for sure.

Offline Toolman

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Re: Antennas
« Reply #33 on: Apr 09, 07, 12:44:39 AM »
Since this post is already titled antenna's I won't start a new thread and people can share their antenna information here.

 I've heard good things of Delta Loop antennas over the years but never had the space to install one. I've been on the receiving end of many loops and they are quite impressive.

 This weekend I completed the installation of my Delta Loop Multi band HF antenna. With the help of Wrightwood, Robert W, Boston Bob and the infamous Johon the tree climber we were able to get about 297' of wire run through the trees at the 35' elevation mark. In my reading and discussing Delta Loop's with other Hams who have used them for HF, the optimum height above ground for Delta Loops is 35-40 feet. The feed point is lower but it doesn't seem to matter much.

 The antenna is fed with approximately 50' of RG-214 coax into a Centaur 4.1 Balun. Centaure makes the best in the business for baluns and they are pricey. The balun was 130 bucks several years ago when I purchased it for another antenna brainstorm that never materialized. A 4.1 Balun is used instead of a 1.1 balun because of the impedance of the length of wire. 450 ohm. ladder feedline could have been used but keeping that stuff away from metallic objects would have been difficult. The wire is supported from the trees with UV resisted guy wire material covered in PVC to ward off the critters. The antenna is almost invisible running around the property. If it were not the mast present for the Balun support, no one would know it's there. The wire has enough flex for tree movement during winds without coming in contact with obstructions.

 Now for the test. It had surprisingly a low SWR given it wasn't cut for a resonate frq. No surprise there as a lot of Delta Loops aren't. I did not need an external antenna tuner, as the internal antenna tuner tunes it up quickly on all bands from 80mtrs. - 10mtrs. including the WARC bands. Which makes me happy because now I've got a good player on 17mtrs. which is where I made my first contact with it.

 Later that afternoon and all the tools put away I sat down for some contacts. With 20mtr.s being my favorite band for DX'ng I started there. I had a few stateside contacts and then came across an opening into Europe. That was a little tougher getting into England and Spain because of the pile up. I'm used to pointing a beam and tossing about 800 watts in one direction. I finally got in and exchanged information. The band died out quickly and moved on to 40mtrs. Another favorite of mine but still haven't got WAS on that band yet. After working several stations with good reports, I think I can easily get WAS on 40 with this antenna in no time. I ran across a three way with a guy in Co. In. and Tx. I joined in with them and they were also giving me some 10 over 9 reports with some 20 over as the band changed. After a while, a few more operators chimed in from back east. Ct. Mn. Ky. and Mi. I chatted with all these guys for about almost two hours and had to peel off for dinner.

 I drew up a map of the tree locations and length of wire between the trees. I plan to visit my buddy at work who works at our onsite antenna range and have him plot my theoretical pattern with his mega buck software. I'd like to see what the pattern and take off angles are....just curious to see what it's doing in space above ground. Hopefully he can provide me a .jpeg like he did for my repeater installation that I can post later.

 While I like and still prefer Yagi's for HF work I found this antenna especially nice because I could hear everyone without rotating the beam to try and find a neutral direction. It's difficult to point toward Mi. and expect to hear a Tx. station in the group with a HF Yagi.

The most expensive part of this antenna was the tree climber.

I think I'm going to like this antenna. ;D


Offline Wrightwood

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Re: Antennas
« Reply #34 on: Apr 09, 07, 01:18:22 AM »
PLus the 6,000 foot tower you put the Delta Loop on didn't hurt.
It's Janos  ;)

Offline Toolman

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Re: Antennas
« Reply #35 on: Apr 09, 07, 03:55:35 AM »
PLus the 6,000 foot tower you put the Delta Loop on didn't hurt.
It's Janos  ;)

 Yes proper validation is in order and my bad.  ;D

If I remember correctly though, we're using the same geographicaly correct tower...in fact your's is a bit higher...... ::)

 Post some of the pics you took whil'st we were having fun impact testing my Dewalt drill from 35'. :-\

KWBoy

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Re: Antennas
« Reply #36 on: Apr 09, 07, 05:57:51 AM »

kew

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Re: Antennas
« Reply #37 on: Apr 09, 07, 06:54:45 AM »


Every thread has to have a complainer  ;D

Offline Toolman

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Re: Antennas
« Reply #38 on: Apr 09, 07, 01:59:07 PM »
Easy Funky.... ;D

 John took some pictures while I was praying my Dewalt would live through the day.

 There isn't realy to much to see since the wire dissapears in the trees, especially at that height. But you can see the mast and 4.1 Balun mounted on the roof.

 He'll get around to posting some after his coffee. ::)

Offline Wrightwood

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Re: Antennas
« Reply #39 on: Apr 09, 07, 02:40:51 PM »
What pressure.........