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-   -   Antenna building party?? (http://www.risingsun4x4club.org/forum2/showthread.php?t=14797)

PhatFJ 01-02-2011 09:52 AM

Antenna building party??
 
I have seen a few threads here about various antenna builds and thought it might be fun to get together and build some antennas. I personally have used my Diamond for mobile, bought an MFJ-1522 dual band for my attic :thumb: and have experimented with a dipole for HF. I was looking at building a good 2meter antenna that I could take to Moab and Lake Powell, would break down well and most of all perform well. I found this on Hamuniverse.com and thought it might fit the ticket, let me know what you think.. http://www.hamuniverse.com/w6jmf2metervertdipole.html.
I have all the tools we would need including an MFJ 269 meter for tuning. Anyhow, I thought I would throw it out there to see if there was an interest.. I am also open to other suggestions..

DaveInDenver 01-03-2011 12:45 PM

That dipole is interesting. Very interesting. You can change it for either horizontal or vertical polarization and for 2m that would be super cool for portable SSB as well as FM.

I have one concern with it, regarding how the dipole legs thread into the PVC and make contact with the brass screws. I understand the reasoning behind this and see the concept, but I would worry about oxidation causing a problem, not to mention each time you thread it together it will want to go a little further. So eventually you either bend the brass screws or the legs are loose in their threads. Either way after a few dis/assemblies the antenna would be mechanically or electrically getting weak.

I'm letting it sink how to make this better without soldering directly to the copper legs. My thinking is towards making something shouldered so that as you screw in the legs you make solid contact with a ring. Just not sure how to do it yet.

Also occurred to me to use 3/4" NPT PVC instead of 1/2" slip fit (e.g. glued to threaded adapter) into the 'T' and add a 3/4-1/2" metal bushing (copper if it exist, this is where you would solder the coax ends). Then the whole threaded socket would be the connection for the legs.

PhatFJ 01-03-2011 07:20 PM

Dave, I had the same concern but thought it might work due to being a limited/portable antenna. I really like the thought of replacing the ends of the "T" with copper fittings and soldering the coax to these :thumb: I will think about this as well and see what we can come up with.. Big thanks for the input!!! We will see if others show any interest...
Talk to you soon...

rover67 01-03-2011 07:21 PM

so what would be the best portable 2M antenna? Is it just a 5/8 wave stick with some radials for a ground plane or something different? Only reason I ask is because I made a J pole just because... I didn't do any research. When I did it was hard to find any real data.

DaveInDenver 01-03-2011 08:38 PM

Quote:

Originally Posted by rover67 (Post 171648)
so what would be the best portable 2M antenna? Is it just a 5/8 wave stick with some radials for a ground plane or something different? Only reason I ask is because I made a J pole just because... I didn't do any research. When I did it was hard to find any real data.

It's a true dipole, each leg is 1/4 wavelength and the overall length is 1/2 wavelength. This was from technician class!!! ;-) Your J-pole is a 1/2 wavelength monopole with a 1/4 wavelength matching stub. Conceptually different types of antennas, although the principles remain. Nothing wrong with any of them, experimentation IS amateur radio. I stopped at Home Depot on the way home tonight and picked up a few bits. It occurred to me that the 3/4-1/2 part I need does come in brass, so I got a couple of them and was going to see how it might solder (doubtful). It might work, though, to drive the brass screws through the reducing bushing, which still accomplishes my goal of making the mating surface the electrical contact, too.

rover67 01-03-2011 09:27 PM

not make each leg a half wavelength or even a full wavelength on that dipole?

maybe that's a dumb question... i'll do some more reading.

Seldom Seen 01-04-2011 09:28 AM

Quote:

Originally Posted by rover67 (Post 171667)
not make each leg a half wavelength or even a full wavelength on that dipole?

maybe that's a dumb question... i'll do some more reading.

A full wave dipole is impossible to feed with coax, the impedance would be way to high. You *might* be able to off center feed it, if you could find the 300 ohm point, then you would be able to feed it with coax through a 4:1 balun. Doing so would give you an acceptable SWR and may show some gain over a 1/2 wave dipole. Dave?????

DaveInDenver 01-04-2011 09:42 AM

I think it would be pretty much impossible to build a practical symmetrical full wavelength dipole. The feedpoint impedance would be very high, my guess is in the >1K ohm range. You'd be thinking 9:1 balun to get anywhere close to 50 ohms and I would still assume you need an antenna coupler. Off center feed would be mandatory!

OTOH, if you make a full length /loop/, that would be a very acceptable antenna. The reasoning behind a full wavelength dipole would be to move the lobes from broadside to the ends, but not much advantage otherwise that I can think.

Seldom Seen 01-04-2011 12:57 PM

Even at 9:1, I still think it would need to be fed with ladder line. Another possibility would be to cut the elements to 5/8 wl and use a 450 ohm matching stub, basically a EDZ. It might show enough gain to be worth the hassle. A 1 wl sky loop sounds interesting, but if I was to spend the $$ on that much tubing I think I'd go with a Moxon rectangle for more gain and directivity.

Groucho 01-04-2011 01:25 PM

1 Attachment(s)
I am assuming the reason for the dipole discussion is to use it for portable situations?

Why not build a cubic quad?
Attachment 22387

or this guy's design?
http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=10840

Or this guy's?
http://www.iw5edi.com/ham-radio/?2-m...uad-antenna,52

Or this video tutorial?
http://www.youtube.com/watch?v=zgQiISj4cc4


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