Multiple bands on 40m dipole

[Radio]

OK, so I'm having a brain f@rt.

I'm making up a 40m copper wire dipole with a 1:1 balun fed with ordinary 50 ohm coax. Now using a tuner, I should be able to get on 40m, 20m, 15m and 10m with reasonable SWR.

Right?

:confused:

[wa8yxm]

Well, 20 may be a problem and so may 10, as on those bands it's an even multiple of 1/2 wave length. Generally speaking a full wave antenna (or 2 full waves) is not a good choice, There is one exception that I know of but generally full waves don't work well

(you get a good swr but at at more than 10 wavelengths from the antenna the ERP is zero)

[k3mp]

Now if you would have off center fed the wire, you would be good for 40 and 15 without a tuner....not quite sure what the tuner would do for you on the other bands

[Radio]

Well, I haven't made the thing yet, so I can offset and trap and so forth to my hearts content.

The goal is to have one simple, cheap antenna that will work 40 - 10.

Ideas?

[wa8yxm]

Traps work very well, can make a 40 mtr antenna into a 20 or 15 or 10 faster than you can say "Shorten the wire"

To tune with traps.. Start with the highest frequency band (10) and put up just the inner most pair of wires and traps. Adjust length of wires for best match near center of desired band.. Then do the next higest band and the next finishing with the lowest (AM broadcast band, right, only we don't check for SWR there, We just eyeball it) Ok, 160 meters

[Radio]

I think I might go the trapless route, where several seperate dipole pairs are attached to the balun and are seperated by PVC pipe, held in parrallel about 5" apart. That's cheap and easy. I have plenty of braided copper wire.

:phone:

[wa8yxm]

That is an option too.. My first antenna was made of twin lead, one side was cut to 80, one to 40 as I recall

[k3mp]

A buddy of mine started his own business on the side of making antennas....I believe he shows how to make these things along with what he sells...just look up KU3X and you will see what he does...I have four of his antennas and quite frankly you can make them cheaper but some times I just get lazy...they do play well...

[Radio]

Thanks. I'll Google KU3X and see what's up.

[Lamewolf]

OK, so I'm having a brain f@rt.

I'm making up a 40m copper wire dipole with a 1:1 balun fed with ordinary 50 ohm coax. Now using a tuner, I should be able to get on 40m, 20m, 15m and 10m with reasonable SWR.

Right?

:confused:

Try feeding it off center 1/3 - 2/3 with a 4:1 current balun and it will work. For more info try google for Off Center Fed Dipole.

[wa8yxm]

Now if you would have off center fed the wire, you would be good for 40 and 15 without a tuner....not quite sure what the tuner would do for you on the other bands

Not correct (Well correct but for the wrong reasons)

A regular 40 meter dipole is 1/4 wave length each side of center on 40
It is also 3/4 each side of center on 15, And thus works just fine.

Dipoles (Center fed) are resonate on every odd harmonic of the design frequency (IE: 40 meters, 40/3 (Which is not 15 but as it turns out is) 40/5 40/7 and so on,

[N3LYT]

A fan dipole works pretty good too.

[Lamewolf]

OK, so I'm having a brain f@rt.

I'm making up a 40m copper wire dipole with a 1:1 balun fed with ordinary 50 ohm coax. Now using a tuner, I should be able to get on 40m, 20m, 15m and 10m with reasonable SWR.

Right?

:confused:

You can get a good match with a tuner, but that does not mean it will work efficiently ! A 40 meter resonant dipole is about 85% efficient on 40 meter, but by using a tuner to get a match to fool the radio, the efficiency drops to about 10% due to the losses in the coax and the tuner caused by a very high swr still present on the coax. You would be far better off not using coax and use balanced feedline that is not effected by high swr like coax is. That 40 meter dipole fed with 300 ohm or 450 ohm balanced feedline would be around 90% efficient on 40, 30, 20, 17, 15, 12, and 10 meters but would need the tuner to do so ! Plus it would have more gain as you move up in frequency since it will be longer than a half wave on the higher bands. Now you could also feed it off center with a 4:1 current balun and get 40, 20, and 10 meters without a tuner. Just make one side around 22 feet long and the other about 44 feet long. This is called an Off Center Fed Dipole and by moving the feedpoint off center the feed impedance stays around 200 ohms where you use a 4:1 balun to convert it down to 50 ohms.

[wa8yxm]

Someone mentioned an OCF dipole woudl work on 40 and 15, As it turns out neither band is EXACTLY that wavelength and thus a 3 to 1 relation exists.

This means a center fed 40 meter dipole works well on 15, But not 10 or 20.