Re: NorCal Doublet


On 1/31/2017 6:21 PM, john adams [4sqrp] wrote:

Just erected NorCal Doublet made of speaker wire as inverted vee.. RST
flat on 20 and 30 with kx1 tuner but can't get it below 4.4 on 40 which
is my main operating band. Any one else had this experience? John, K4QQ
I think the original NorCal Doublet was made from 4-conductors of a computer ribbon cable. Since the outer two conductors are used with the inner two spacing the conductors apart your speaker wire probably has a vastly different impedance. That may not matter for your question.

Did you cut and tune the antenna for 40M before using the tuner since that's your favorite band? I'd start by putting the tuner in bypass mode and adjusting the SWR on you favorite 40M frequency first and then let the tuner take you other places as needed.

This method gets dipoles where you want them with a minimum of fuss:

Dipole Calculations -- 2 Adjustment Method.

F = target frequency
L = length of dipole in feet
M = "magic" formula constant for 1/2 wave dipoles

L = M / F

Example F is 7.150 MHz (middle of 40 Meter band)

L = 468 / 7.150 = 65.45 ft = 65 ft, 5.4 inches

Build the dipole to this length. Leave the ends temporarily connected so the length is easily adjusted later.

Install the dipole and measure the SWR across the band. Note the frequency of the lowest SWR.

For discussion purposes lets say our SWR minimum was measured at a frequency of 7.20 MHz. The adjustment needed is:

Adjustment = F(SWR minimum) / F(target) = 7.200 / 7.150 = 1.007%

Our wire length needs to be 1.007 * 65.45 = 65.91 = 65 ft, 10.92 in.

If you adjust the wire length to this new length the lowest SWR should be exactly where you want it.

Why is the length different than the "standard" formula? "468" is the constant needed to calculate the length of the dipole under perfect conditions with a given wire size, no insulation, etc. You did not use those conditions and materials for your dipole. Using the above method compensates for not knowing all of the technical details of your feed line, antenna wire, etc.


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