Dipole Impedance - I must be slow!

[W1AJO]

I've been trying to figure the impedance of a dipole at the feed point. I have found lots of formulas and theory that I don't really understand - I must be slow. Just need a simple answer.

I'm building a 20m dipole that will be coax fed. The antenna will be tuned to 14.060 MHz using a MFJ 259B.

I plan to use a balun at the feed point (at the top of the coax where it joins the dipole legs) but I can't find a reference that tells just what the impedance of a dipole is at that point thus letting me figure out what value balun I need to match it to a 50 ohm coax feed line.

I'm a bit anal about this antenna as I need it to be as efficient as possible and the SWR needs to 1:1.0. The rig is a Rockmite 20 that puts out a whopping 1/2 watt so I cannot afford to lose any power.

Thanks!

[K7JEM]

A dipole has about 72 ohms impedance in free space. When mounted within a few wavelengths of ground, the impedance varies somewhat, depending on height.

A dipole about .5 wave above ground will be fairly close to 50 ohms. In any case, the SWR on a dipole should be easily under 2 to 1, which is good. You do not have to get perfect SWR in order for the antenna to be effective and efficient. 2 to 1 SWR is about 11% reflected power, that much loss will never be noticed at the RX end of the QSO.

Joe

[W1VT]

I suggest chapter 3 of the 22nd Edition of the ARRL Antenna Book--There is a chart that shows the variation of feed point impedance of a half wave dipole with height above ground. This assumes a flat top dipole. Mounting the dipole in a vee or inverted vee dipole typically complicates the equations to the point where it makes more sense to either measure the impedance or model the antenna on a computer.

[KB5UBI]

I know this may sound a bit complicated but it is very simple to build a single band dipole with a DC grounded center fed with a 4:1 coax balun at the 200 ohm points on the dipole. The 200 ohm points are easy to find with an analyzer or portable SWR bridge. The dipole can be shortened a bit with the addition of a hairpin with it grounded to the boom.

[WB2WIK]

I'm a bit anal about this antenna as I need it to be as efficient as possible and the SWR needs to 1:1.0. The rig is a Rockmite 20 that puts out a whopping 1/2 watt so I cannot afford to lose any power.

Use a 1:1 balun.

Any reflected power not absorbed by the transmission line gets radiated anyway. If you have a short feedline, you lose basically "nothing" with a mismatch as long as the transmitter can transfer power into it.

[K8ERV]

ant height z.jpgI found this in my info pyle.

TOM K8ERV Montrose Colo
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[KJ3N]

I'm a bit anal about this antenna as I need it to be as efficient as possible and the SWR needs to 1:1.0.

A 1:1 SWR is no indication of antenna efficiency. A dummy load has a 1:1 SWR. Doesn't make it an efficient antenna.

Almost none of my wire antennas has a natural 1:1 SWR, yet they work quite well.

[W3WN]

I've been trying to figure the impedance of a dipole at the feed point. I have found lots of formulas and theory that I don't really understand - I must be slow. Just need a simple answer.

I'm building a 20m dipole that will be coax fed. The antenna will be tuned to 14.060 MHz using a MFJ 259B.

I plan to use a balun at the feed point (at the top of the coax where it joins the dipole legs) but I can't find a reference that tells just what the impedance of a dipole is at that point thus letting me figure out what value balun I need to match it to a 50 ohm coax feed line.

I'm a bit anal about this antenna as I need it to be as efficient as possible and the SWR needs to 1:1.0. The rig is a Rockmite 20 that puts out a whopping 1/2 watt so I cannot afford to lose any power.

Thanks!

If you hang it as a true in-the-plane 1/2 wave dipole, the impedance at the feed point will be roughly 72 - 75 ohms.

If you hang it as an inverted "V", the impedance at the feed point will be roughly 50 - 52 ohms. That's assuming that it's a true "V" with the legs 90 degrees apart.

There are other factors involved... type of wire, velocity factors on the coax, and so on. Rule of thumb is to cut it a little on the long side, based on the usual formula calculations, and then trim evenly from each end until you se the SWR you want on that frequency.

Good luck!

73

[K8ERV]

Heck, I'm still trying to figure out the impedance of that wet string.

TOM K8ERV Montrose Colo

[K8JD]

Overcomplicating a simple dipole is futile ! Just hook up the coax at the center and get the thing up as high as possible and it can't help but radiate ! You don't really need any stubs/hairpins/baluns/tuners to make a simple, monoband dipole work. "Gilding the lilly" is the phrase that comes to mind.

I know this may sound a bit complicated but it is very simple to build a single band dipole with a DC grounded center fed with a 4:1 coax balun at the 200 ohm points on the dipole. The 200 ohm points are easy to find with an analyzer or portable SWR bridge. The dipole can be shortened a bit with the addition of a hairpin with it grounded to the boom.