How Can A Fanned Dipole Only Use One Feedline?

[N4CR]

The interesting thing is that even though some current will flow on the unmatched sections, that current does cause radiation, so it's not a loss.

[K8JD]

RF is AC and the rules of physics are different for AC and DC. Go back and read your basic antenna and feedline section in the "Radio Amateur's Handbook" ! Review Impedance and reactance info also. Most of the answers to questions in this forum are in that book ! Find an "Elmer" that works in the RF field to sit down with and open the "Handbook" together.
The "Handbook" and FCC Amateur Service rules should be in every HamShack !

If I test copper wire on a multimeter, it will usually show almost 0 ohms of resistance and the length of wire makes very little difference. So how can it be 50 ohms on a radio? Or does the resistance change when you change frequencies on the radio?

I guess I just don't understand how if you hookup several wires of different lengths to the same coax line, how the radio signal "knows" to only use one of those wires and not the others. Won't the radio signal just go into all the wires?

[KB9BVN]

The radio doesn't "know" anything. It's called "resonance". The length of wire that is a resonant length for the band being transmitted on is the element that will radiate the signal. The other adjacent elements will also modify the pattern due to their proximity to the radiating element, but not enough to cause much distortion of the pattern.

With a fixed antenna, like a dipole, the thing is typically only resonant at one specific frequency. So this fan dipole working is more about impedance than where is it resonant.

[KE3WD]

The radio doesn't "know" anything. It's called "resonance". The length of wire that is a resonant length for the band being transmitted on is the element that will radiate the signal. The other adjacent elements will also modify the pattern due to their proximity to the radiating element, but not enough to cause much distortion of the pattern.

Exactly.


Recommend that the OP study up on the physics of "resonance".


73

[WA9SVD]

The radio doesn't "know" anything. It's called "resonance". The length of wire that is a resonant length for the band being transmitted on is the element that will radiate the signal. The other adjacent elements will also modify the pattern due to their proximity to the radiating element, but not enough to cause much distortion of the pattern.

Exactly. The "resonant" dipole will present a reasonable match to the transmitter. (typically, between approx. 36 and 100 Ohms IMPEDANCE.) That is the portion that will (for all practical purposes) radiate the RF signal. The other dipoles cut for different frequencies, will present a high impedance to the transmitter. Even though we are concerned with impedance, apply Ohms Law: consider that a "50 Ohm" impedance in parallel with a 1000 Ohm Impedance. There will be very little current flow into the 1000 Ohm impedance, and there will not be a significant reduction in current flow in the resonant "50 Ohm" antenna.

Now one problem that can occur is with multiple dipoles if one is designed to resonate on 40 Meters, and another is designed for 15 Meters. BOTH will have relatively low impedance on 15 Meters, and will present a poor match on that band.
For that reason, if a multi-dipole antenna is contemplated starting with 40 Meters, a 15 Meter dipole should NOT be included. The 40 Meter dipole will function as a 3/2 wave antenna.

[WA3UCR]

Some good responses here. The concept of Impedance is typically a tough one for a new ham or a new student of electronics to grasp. But, it is not that tough. Because RF is actually AC, we must first learn about the concept of Reactance. Reactance comes in two forms. Inductive (Xl) and Capacitive (Xc). Both of these are frequency dependant. Reactance is measured in Ohms.
Xl = 2 X Pi X F X L where F is in Hz and L is in Henries.
Xc = 1/ (2 X Pi X F X C) where F is in Hz and C is in Farads.

Pick a value for Capacitance or Inductance and watch how the Reactance changes as you vary the frequency.

Now, when we bring Resistance into the mix along with Reactance the resultant is Impedance. Impedance is the vector sum of both the resistive and reactive componets at a given frequency. This is a basic, simplified explaination - there is a lot more to it ( parallel, series components, etc). Then, you can start investigating the concept or Resonance, or where the system voltage and currents are in phase. There are lots of good tutorials on this subject available on the internet.

It's cool stuff, but takes some study to get your head around the concepts at first. Good luck, I hope you pursue it.
73
Bill

[G3TXQ]

A couple of observations:

1) Glen's plot of dipole resistance vs height applies over perfectly conducting ground! Over average ground, at low heights the resistance doesn't drop below about 30 Ohms.

2) At the feedpoint, more current flows into the dipole with the lowest impedance, not the one with the "best match" - simple Ohm's law. If you could rig a situation where one dipole was 50 Ohms and another was 25 Ohms, more current would flow into the 25 Ohm one than into the 50 Ohm one.

73,
Steve G3TXQ

[W4HAY]

...Now one problem that can occur is with multiple dipoles if one is designed to resonate on 40 Meters, and another is designed for 15 Meters. BOTH will have relatively low impedance on 15 Meters, and will present a poor match on that band.
For that reason, if a multi-dipole antenna is contemplated starting with 40 Meters, a 15 Meter dipole should NOT be included. The 40 Meter dipole will function as a 3/2 wave antenna.

Putting this into practice, the temporary sloper I have up (thanks to my neighbor's Pine tree falling across my big antenna) is a 40 Meter dipole with a 20 Meter dipole slung under it. The SWR on 40, 20, and 15 is under 1.8:1 across the CW portions of all three bands. Gets out well, too!

[WD4CHP]

I will probably get a 1 month suspension for this BUT I don't care.

You an EXTRA.

I learned about impedence when I was a novice!

[K8MHZ]

I will probably get a 1 month suspension for this BUT I don't care.

You an EXTRA.

I learned about impedence when I was a novice!

Too bad you never learned how to spell 'impedance'.



(Sorry, that was just way to tempting to let slide!)