20 meter dipole question

[North Texas Mudduck]

say if i am going to rig up a 20 meter dipole
center freq at 14.250
so that would tell me each leg would need to be 16' 5/16"
correct me if im off
and i need to give a little more for tuning or cutting purposes

now say if i measure a leg at 16' if i found the center of that 16 foot leg we know that would be 8foot
now im wondering if i put the 8 foot center at the balun and ran out 8 foot in a V direction horizontal

and also with the other leg at 8 foot going out at a V pattern in the horizontal

would i still be in the 20 meter center or before i go cut up some stainless steel cable
would i be in the 10 meter range

if might look something like this


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[wavrider]

Not sure how the current distribution on that would be.


An antenna is fed at a current or voltage point to try and get a 50 ohm impedance, IE the OCF dipole.

 

[North Texas Mudduck]

some i have talked to are calling it a X beam or X antenna

 

[W5LZ]

The length of a single 1/2 wave dipole at 14.250 Mhz is 32' 10", or 16'5" on either side of center. I'd add enough length to that so that you can make connections and still end up at 16'5" per side. That length is still subject to tuning so give your self enough 'extra' length to play with.
If you want to add a second 1/2 wave dipole to that antenna, or branch it off at the 8 foot point on both sides of center, that ought'a work too. That 'Y'ing in effect is the same as using a much larger conductor than just wire/cable, it's not uncommon. The tuning is still a matter of adjusting length for resonance, and then matching impedances. That impedance matching is commonly done by brining the two sides of a dipole closer together, something like 120 degrees between legs instead of 180 degrees. I don't see why it shouldn't work for you. You will end up with a center fed dipole antenna, NOT an OCF antenna. Treat it like a dipole and it should act the same as one.
- 'Doc

 

[KC9Q]

Hi North Tex,

If you had the dipole ends seperated by 3 to 4 ft, with the dipoles oriented vertically (sort of like a fan dipole), you would have a wideband dipole design. On the other hand, if you had the dipoles seperated by 90 degrees in the horizontal plane (i.e., a horizontal 'X') you would have a crude Turn-Style antenna that would provide a rough omni-directional horizontally polarized pattern. The impedance for either design would provide something close to 40 ohms (+/- 5 ohms). You could further enhance the broad band capability by building one dipole for 14.100 MHz, and the other for 14.300 MHz. Make the wires a little longer then calculated to allow tuning at your particular QTH. Build it and see what happens.

 

[Captain Kilowatt]

It's nowhere near 20m guys. He has the middle of each 20m length connected to the balun essentially making a pair of 10m elements on each side. It's not really a broadband dipole for 10m nor a Hexbeam for 10m. IMO it is a mistake.

 

[wavrider]

Doc you might want to look at the drawing again???

it is a 1/2 wl element fed in the middle!!!!

That makes it a 1/4 wl on each side.

CK said exactly what it is.

It would be mostly an omni directional radiation pattern resonant somewhere near 28 mhz.

As far as the four leg dipole, very common on 75/80 meters so the full band can be covered.

The length of a single 1/2 wave dipole at 14.250 Mhz is 32' 10", or 16'5" on either side of center. I'd add enough length to that so that you can make connections and still end up at 16'5" per side. That length is still subject to tuning so give your self enough 'extra' length to play with.
If you want to add a second 1/2 wave dipole to that antenna, or branch it off at the 8 foot point on both sides of center, that ought'a work too. That 'Y'ing in effect is the same as using a much larger conductor than just wire/cable, it's not uncommon. The tuning is still a matter of adjusting length for resonance, and then matching impedances. That impedance matching is commonly done by brining the two sides of a dipole closer together, something like 120 degrees between legs instead of 180 degrees. I don't see why it shouldn't work for you. You will end up with a center fed dipole antenna, NOT an OCF antenna. Treat it like a dipole and it should act the same as one.
- 'Doc

 

[W5LZ]

Hmm, that's not how I read it. That, "would tell me each leg would need to be 16' 5/16"", sounds like 20 meters if it was 5" in stead of '5/16", so just figured a typo. That 'spreading' of each 'leg', or parts of a leg also sounds like a Collins variation of a dipole to make it usable over a wider frequency range.
- 'Doc

 

[Captain Kilowatt]

The middle of EACH 16 foot length is connected to tbe balun making it a pair of 8 foot elements. A turnstyle for 10 m maybe?

 

[BJ radionut]

CK: I read it the same as you have...Mudduck is "Folding" the 20m
1/4 wave element and feeding it in the middle...(2) 8ft legs....

Mudduck not quite sure what your trying to achieve by doing that?

Even at 10 mtr's it may not be usable.
I was always under the impression when trying to achieve wide bandwith, (1) or (2) 1/2 wave elements were placed in very close parallel proximity to the driven dipole....However they are not connected at the feedpoint...

Seems like I remember and old Tri-bander design where the 20m element was "excited" and the 15m and 10 meter elements spaced very close but not connected to the coax to avoid using traps on the driven element. These elements were isolated from the boom to achieve this???
Or maybe I'm having a "senior" recollection:laugh:

I have seen Doc's thoughts (I think) in regards to a set of sloper type verticals used on 80/75 mtr's using the same feed point with one leg longer than the other to increase bandwith...Seems like I have and old handbook here from the 50's, that does a similar idea with 160 mtr's

Again I guess Mudduck what are you trying to accomplish by the use of this "X" Factor???
All the Best
Gary

 

[Captain Kilowatt]

Gary you are correct about the tribander. They called it "shock excitation" . The Hygain Explorer 14 uses that for 10m. The driver element is directly driven on 20m and only has a 15m trap. There are two 10m dipoles about 6 inches either side of the main driver element. Hygain also uses that idea on the DB-1217 where the 17m driver is directly driven with dipoles foe 12m. IIRC it is a form of sleeve dipoles.

 

[North Texas Mudduck]

reason for folding each leg in and making 2 legs is to save some space
and so with some further research
even if i did the 4 legs each leg would still need to be the 16 foot and some inches

 

[Captain Kilowatt]

A 20m dipole is approx. 16 feet from center insulator to the end making it approx. 32 feet end to end. Anything less is not tuned to 20m unless it has lossy loading coils in it. You can't change the laws of physics.

 

[wavrider]

reason for folding each leg in and making 2 legs is to save some space
and so with some further research
even if i did the 4 legs each leg would still need to be the 16 foot and some inches

You can fold back the legs, just keep some space between them, this will narrow the band width and effect the radiation pattern but it will make for a smaller foot print

 

[W5LZ]

The other method of 'shortening' an antenna is by using a 'capacity hat'. An antenna can't be shortened a lot by using a 'cap-hat', but it can be shortened. (Or, another way of thinking about it is that the frequency of that antenna it lowered a bit, same result just a different 'view' of what's happening.) All things considered, the total shortening that happens would only be something in the neighborhood of a foot or two, maybe? Not something to be that useful unless you're really strapped for space.
It also has the 'benefit' of making the antenna slightly more 'broad-banded'. It's an odd way of looking at it but think of resonance as a 'quantity' of stuff. It can be 'thinned'/'diluted' or made 'softer' until it get's to the point of not being there at all, or not worth the trouble. 'Hard' resonance' and 'soft' resonance. Howz that for being 'escher-istic'?? (I said it was 'odd'!)
Ever seen/remember seeing a dipole with large metal disks on the ends (pie plate size)? That was another variation on this sort of thingy. Not very useful and the idea didn't stick around too long.
The biggy with any of this is how practical is it? The 'split leg' thingy with dipoles is a little useful, but not a lot. It can 'broaden' the antennas 'resonant-width' a bit, but physics still is in effect so don't expect miracles. Think of it as an experimentation in the resonance of 'shapes'?
- 'Doc

(I'll probably wish I hadn't typed any of this after I get another cup or two of coffee in me...)