How to estimate a dipole feedpoint impedance?

Question

An ideal dipole, at resonance, will have an impedance around 73 ohms. A folded dipole, around 280 ohms.

How can I calculate the impedances when not at resonance? Let's assume I have a span of 15M in my property where I can hang a dipole. Ideally, and very roughly, a folded dipole would give me about 280 ohms impedance at 10MHz.

How can I estimate what the impedance will be at 160, 80, 40, 30, 20, 17, 15, 10 M will be?

I want to build a balanced matchbox and I want to estimate the required L and C sizes, to figure out what the range of matching would be.

Answer 1

Read it off this chart:

Interferometrist [CC BY-SA 4.0], via Wikimedia Commons

A folded dipole with equal-sized conductors has 4x the impedance of an ordinary dipole.

The impedance repeats every wavelength, so for example if you're interested in the impedance at 2.5 wavelengths (off the chart) it won't be far off from the impedance at 1.5 wavelengths.

Height above ground, conductor diameter, and non-straightness of the dipole elements can alter the impedance, but if you just want a rough estimation this will do just fine.

Answer 2

There are at least two ways. Firstly, you can use the antenna modeller (I prefer cocoaNEC) to estimate an impedance of the antenna at given frequency. From personal experience it doesn't work that well though. The real antenna will differ quite a bit from the model because of unknown ground properties, various metal object near antenna, etc. This method may still be useful, e.g. if the antenna is erected very high.

Secondly, you can use an antenna analyzer (I use EU1KY antenna analyzer, model FFA-450) to measure the impedance. This method is not perfect as well, especially if impedance is high, but it gives numbers that are close to reality. I choose L and C using the impedance measured buy the antenna analyzer and wind a few extra turns in the coil. Then I tune L by stretching the coil until I get SWR close to 1:1.

(UPD: By the way, I have a very pleasant experience of using a delta loop under similar conditions. You can find a diagram here and the full description in Russian here).