10 meter dipole questions. (My 1st build)

[jassing]

I am building my 1st dipole, and wanted to be fairly basic.
I get the jist of most of the 'how to' and lengths.
My question is: instead of the wire being pulled 'straight out' to the sides, could it be gently wrapped (to shorten physical length of the arm, not the wire length) (say 1 wrap per 3") around a pvc pipe so it could be more easily turned (direction)? Or does the wire need to be as straight as possible?

Has anyone made a air-choke ("ugly balun") -- the idea seems pretty simple, and i've read about a dozen different "my version is better" type pages; but they're all the same.
Two things are not so clear.
1) Must the air balun "tube" remain vertical/inline with the feed coax?
2) Can the air-choke be "suspended" around the antenna mast ? (so the wraps are around the pvc pipe, not touching the mast, but the mast is going up thru the center of the pvc pipe)

 

[ko6jw_2]

A 10 meter dipole would be around 16 ft. plus or minus. Why would you want to shorten what is already a short antenna? A helically wound antenna is certainly possible, but most designs I've seen are for short verticals. The length of wire and the turns ratio need to be determined and the support will have an effect on the antenna. Thus, you will need to experiment should you want to build such a thing.

The air balun does not need to be aligned in any certain way. It can be suspended. It can be wound with no core.

If horizontal space is an issue, why not consider an inverted Vee?

 

[jassing]

Thank you for the feedback.

Everything is relative... what looks small in parts in the garage, looks huge put together with the wife looking on. Moreover, the space available for a horizontal antenna that can turn is pretty limited when considering trees, wind, flex etc...

I know some antennas are wound (my cb antenna is wound the full length) -- I wasn't sure if it was a tuned thing where the turns need to be precise, just wasting wire, or only good for some wavelengths.

At about 12-14' I got an approval from the wife & landlord, which is why I was hoping I could gently wrap them with keeping the electrical length and not 'looping' the wire ..

I wasn't overwhelmed with what I read on inverted V's performance; but perhaps if I weigh it against reducing performance with a horizontal by wrapping the wire, maybe a v will make more sense. I'll revisit the inverted V (thanks). The space that remains for a turnable antenna is limited, I have to take into consideration the trees, wind, and flex -- I just don't have a lot of space (another reason for shortening the physical length).

I was actually going to make two, one for horizontal and one for vertical and just switch between the two as needed/desired. No rotor yet, so I am not really in a hurry to slap something together.

Thanks for the input on the air balun. I was actually going to do it "inside" a 4" pipe to keep it really clean; which is why I was hoping I could support it with the antenna mast thru the center; then it would look really clean.

This is my 1st antenna build -- in my youth all my cb beams & antennas were 'real' antennas from hand-me-downs & 'built out of spare parts'; but the loss of the tower, beam, & omni hit hard & it is looking like insurance won't cover it (We own the house, rent the land, they are screwing with us on a technicality that the antenna wasn't part of the house & therefore not part of the policy, we would have needed renters insurance for anything "not attached to the house") -- We are fighting it. In the mean time, I need to get some antennas up on the cheap before winter sets in...

 

[ko6jw_2]

I think you are making this more complicated than it needs to be. If you are trying to work DX on 10 meters the polarization is more or less irrelevant. An inverted Vee will give you a mostly omnidirectional pattern (there are some limits like the effect of the mast). Given the small foot print available it might be a good choice.

I think that sometimes operators forget that getting on the air is the main point, not some antenna theory. Properly designed you won't need a tuner for just ten meters. I made my first 10 meter contact with a dipole fed directly with coax (no balun) strung a few feet of the ground on my patio. With 12 watts I talked to a guy in Argentina. Later I put the same dipole on the roof and made many contacts. Of course, 10 meters was hotter then than its is now, but there are still openings.

Your living situation sounds like it is making things complicated. You could also look at an end fed wire with a tuner. A random length run to a tree could work with the right tuner. There are issues with end fed antennas, but properly made they can work very well. You will need a good RF ground at the tuner to keep RF from coming into the shack.

 

[SCPD]

Bend the ends

Winding the calculated length of a dipole around a core, like what you want to do, isn't going to make it resonant anymore.

I think it would be more effective if you ran as much of the antenna going from the center horizontal, and then bend the two ends down at 90 degrees.

 

[chrissim]

See this link for chokes: Common-mode chokes

According to the ARRL Antenna Book, you will lose about 6db using an inverted vee configuration opposed to a dipole whose elements are relatively horizontal. "There is a small loss in peak gain and the pattern is less directional (p. 9-5, 22nd edition)" Dipoles can be erected in a myriad of configurations. The first 60% of the elements are the most important, you can let the rest hang down vertically on each side if necessary, but I don't think I would wrap the elements around anything.

As for the choke, I've always used them on dipoles, even though many argue it isn't necessary - and it may not be depending on your conditions. However, to reference the antenna book again: "Adding a choke or current balun at the feed point helps to electrically isolate the shield surface and prevent common-mode current from flowing on the feed line (p. 9-2, 22nd edition)" MFJ makes a decent and cheap solution, the MFJ 918. It's about $25.00. I normally take mine apart and restructure it so that I can use it over and over again on different projects. It's effective enough.

If you're going for the "ugly balun" solution, make sure you reference the link at the top. Just randomly winding coax won't do much.

 

[K7MEM]

jassing,

A shortened antenna is certainly possible, as long as you realize that the efficiency will go down. So here is a possibility to think about.

Let's say we are designing a shortened dipole for 28.5 MHz and your maximum horizontal space is 14 feet. A Full Size dipole for 28.5 MHz would require 16' 11" (5.157 M). For maximum efficency, the Shortened Dipole should be no shorter than 11' 3-15/16" (3.453 M). So 14' is a practical length. To see what the possibilities are, you can use the following URL:

Martin E. Meserve - K7MEM - Electrically Shortened Center-Fed Half-Wave Dipole

Under "Initial Parameters" you can enter antenna center frequency (28.5 MHz), the available space (14'), and then the wire size to be used. The "Possible Configurations" section will then show you 10 possible configurations, based on your input parameters. You should see selection "5" highlighted. The selections are listed from "worst" (0) to "best" (9). The idea here is that selection "0" has the inductor close to the feedpoint (base loaded) and is the least efficient. At the other end, selection "9" has the inductor is at the ends and is the most efficient. But what you should also notice is that as you move the loading coil away from the center, the inductance gets larger. Often it gets too large to be practical. Selection "5", center loading, is then considered a good tradeoff.

For the specifications listed above, selection "5" says that you will need a 1.0 uH inductor that is 1-3/8" (34 mm) in diameter. To see what that means in "number of turns" you can go to another web page at:

Martin E. Meserve - K7MEM - Single-Layer Air-Core Inductor Design

Under the "Initial Design Input" section you can enter your required parameters, 1.0 uH, wire size and wire type. It will initially design a optimized inductor. But the inductor's diameter (0-3/4") may not match your required diameter. Let's assume you are using 1-1/2" PVC (actual OD 1.900") for the antenna and want to wind the coil right on the PVC. In the next section, "Alternate Form Size Selection" you can specify the diameter you require and a new inductor will be created, based on the 1.900" diameter. The new inductor will require 3.8 turns spaced ~1/8" apart. While it isn't required, in the next section you can adjust the inductor for a even number of turns (4 turns). This might help simplify the measurements.

There is a lot of abiguity in the available information on baluns. It's hard to decide who is correct and who is not. One source recommends that you just wind coax like a rope and another source says you need to use nice even windings. Who's right? My guess is they are all right to some degree.

Personally, I use the old standard dipole balun. These are usually 1:1 baluns wound on ferrite rods and look like the balun pictured in Martin E. Meserve - K7MEM - Center-Fed Half-Wave Dipole ( 3-30 MHz ). I find these baluns regularly at ham fests for $8 to $10 and they work fine. As long as your coax feedline travels away from the center of the dipole at a 90 degree angle, there shouldn't be any feedline radiation to worry about.

Good luck on your antenna.

 

[jassing]

wow! Thank you all!!! great stuff. I don't think I would have picked up all this just reading endless random websites.

I like the 60%, or 90degree turn downs at the ends -- that is a great solution and will try that. If that fails, I'll reconfigure to try an inverted V

Thanks folks!

 

[wb6uqa]

10 meter dipole

You can use two ten meter Ham Sticks in a dipole . Most radio stores sell the hardware. This will make a very short antenna. The impedance of a dipole is around 70 ohms. I would use RG59 coax without a ugly balum.

 

[chrissim]

wow! Thank you all!!! great stuff. I don't think I would have picked up all this just reading endless random websites.

I like the 60%, or 90degree turn downs at the ends -- that is a great solution and will try that. If that fails, I'll reconfigure to try an inverted V

Thanks folks!

jassing: there is no reason why allowing the ends to hang will not work. I promise, it's a proven solution. Be sure to prune each end evenly when tuning. Generally, you'd like to get the antenna at 1/2 to a full wave in height if you're considering DX work. This translates to about 35 feet for a full wave at 28.300. If my math is wrong please correct.

In terms of the balun question: please take a look at an informative write up by W8JI, available from DX Engineering's webpage :

Baluns: Choosing the Correct Balun - Free Shipping on Most Orders Over $99 at DX Engineering

In short: use a 1:1 current choke.

I really want your first antenna project to be great. Have fun and don't rush it : )

 

[prcguy]

A 10m hamstick is usually 7ft long but a full quarter wave on 10m is only about 8ft, so what do you save by going with hamsticks?
prcguy

You can use two ten meter Ham Sticks in a dipole . Most radio stores sell the hardware. This will make a very short antenna. The impedance of a dipole is around 70 ohms. I would use RG59 coax without a ugly balum.

 

[jassing]

Thanks for that. I will take my time & trim evenly - being my 1st I am probably going to be overlay cautious and trim slowly. I should be able to get 35' above ground, BUT: we are on a steep rock/hillside so even tho I may be 35' up the pole, it won't be 35' above a horizontal view... Trying to do the best with the limitations we are presented with. What is it the marines say "Improvise, Adapt and Overcome".
I've always had store bought antennas with 1 exception in the 80's I bought a home made 3 element beam for cb that worked awesome, so I know it can work; especially with the help of others who've done it.
Thanks!