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View Full Version : Antenna building party??


PhatFJ
01-02-2011, 10:52 AM
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, 01:45 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, 08:20 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, 08:21 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, 09:38 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.
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, 10:27 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, 10:28 AM
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, 10:42 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, 01:57 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, 02:25 PM
I am assuming the reason for the dipole discussion is to use it for portable situations?

Why not build a cubic quad?
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-meter-cubical-quad-antenna,52

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

DaveInDenver
01-04-2011, 02:32 PM
I don't think balanced feedline would be necessary and for VHF open 450 ohm ladder would be bad, more so if the air was moist. Using 300 ohm TV feeder would be an option, though. In particular to avoid tons of loss at >144MHz the conductors would have to be close, others it's no longer balanced since they don't couple and just act like independent wires.

Probable that I'm missing something, what are you trying to solve using the balanced feed here Brian? If you feed an unbal 50 ohms into a 9:1 balun into 450 ohm ladder, which you connect to a high impedance, you still get a huge reflection and essentially make the ladder line the radiator. So yes, it's keeping you from blowing the radio finals but doesn't solve the feedpoint inefficiency.

I 100% agree that solving a full wavelength dipole problem is not worth the time and doing a matching stub or changing to a 5/8 radiator makes a lot more sense. :-)

BTW, teachable moment... If you look at the construction of this antenna, you'll see the author wrapped the coax several times at the end. He's making a 1:1 current balun to solve the balanced-to-coax issue. In this case being a correct Hertzian dipole the feedpoint impedance will be ~72 ohms, so you don't need the impedance transformation that Brian and I are discussing.

Seldom Seen
01-04-2011, 03:04 PM
'Cuz even with a 9:1 balun you are still looking at an impedance miss match of around 3:1. Your transmitter would tolerate it but coax would become very lossy.

DaveInDenver
01-04-2011, 03:10 PM
'Cuz even with a 9:1 balun you are still looking at an impedance miss match of around 3:1. Your transmitter would tolerate it but coax would become vary lossy.
That was my more or less my guess. I think there really isn't a practical solution to this for high band VHF because the ladder line becomes lossy at those frequencies. Darned if you do, darned if you don't. My thinking is that a wound transformer would be less lossy than any cable, but then a 9:1 balun is a basically a great big lumped heater that leaks a little RF, forgetting 20:1. Really 4:1 is about the limit of practicality. On the up side the antenna would be resonant at least, if almost impossible to feed. So the antenna itself would be efficient at radiating whatever makes it down the pipe.

PhatFJ
01-04-2011, 06:22 PM
Wow, I am glad to see all this discussion!! Basically, I have never built an antenna from scratch, which I can only assume other new Ham operators have not. I was interested in a design which is portable, breaks down and sets up easily, does not take up much room and works well. I was thinking if it can break down and be put inside of a larger piece of PVC tubing with end caps, that would be tough, durable and easy to pack. If we can get enough interest I am happy to volunteer my house for assembly, I have all the tools necessary to put the antennas together and the MFJ-269 for tuning.. I am in the South West area Simms & Belleview.. Kick this around some more and lets see if we can come up with a design that everyone likes and would like to try, then we can put together a list of materials and work on a date and time.. :thumb: Just a note, I for one get just a bit intimated with the level of expertise and knowledge you guys have and want to start grabbing some books to look up some of the things you are saying :bowdown: I think it is GREAT and I hope others can take advantage of this generous knowledge and willingness to help..
So: THANKS!!!

CardinalFJ60
01-04-2011, 06:30 PM
I need tO remove an old UHF/VHF tv antenna from my house. Would those materials be useful tO donate to the cause?

CO Hunter
01-04-2011, 06:44 PM
If it is a weekend I would be tempted in coming up from the Springs. Since you're offering your house I would understand if you want to keep the group small. I'll keep watching this post. Great stuff, not that I get it all.

PhatFJ
01-07-2011, 09:54 AM
I need tO remove an old UHF/VHF tv antenna from my house. Would those materials be useful tO donate to the cause?

I do not know if we could use any of these materials or not but thanks for the offer..

If it is a weekend I would be tempted in coming up from the Springs. Since you're offering your house I would understand if you want to keep the group small. I'll keep watching this post. Great stuff, not that I get it all.

I would like to do this on a weekend, I think there would me more interest and more time available. Let's see what the level of interest is and we will go from there..