December 22, 2008

An 80m/40m Dual Band Vertical

With solar conditions being the way they are right now, I decided that putting up a decent antenna for 80m and 40m would be a good idea. Things need to be kept low profile here but I am fortunate to have a 90' tall oak tree to work with.

After pondering a few ideas for a good performing antenna to use on 80 and 40, I recalled an article from September 2005 QST by Dan Richardson K6MHE about a dual band 40m and 20m vertical described previously by Wes Hayward W7ZOI . By using a modified L-network, the antenna appears as a regular 1/4 wave vertical on 40m, but has a shunt capacitor to match the very high impedance of an end fed 1/2 wave vertical on 20m. Two bands, one antenna, switched automatically. Nice!

A copy of the article can be found here:

After reviewing the article, I decided to give a 80m/40m version a try. Parts for the matching network were salvaged from an old MFJ antenna tuner. I wired the network together and did some test on the workbench with my AIM4170 impedance vector analyzer. For convenience and future changes I decided to leave the inductor rotary switch mechanism attached as removed from the tuner. See diagram below. By positioning the switch for the best tap on L1 and adjusting C2, I found that the L-network would work satisfactory down to 80m. Tuning was done empirically after the antenna was constructed. L1 ad C2 are adjusted to be a low impedance path on 80m and shunt C1 is adjusted for resonance on 40m.

The network was assembled into an outdoor electrical junction box that can be found at Lowes or Home Depot. Due to my limitations on antennas, only 4 68' #14 solid radials could be used. The vertical radiator is 68' #10 stranded running nearly vertical up to a pulley from a branch in the oak tree which allows me to raise and lower the antenna. The radiator is a little longer than needed on 80m being resonant around 3.45MHz but ends up falling resonant right in the middle of the 40m band. I figured that I would rather have a higher SWR using coax on a lower frequency than a higher one. Further adjustments to the network did not improve this much. I used the same type 20-300pF air variable capacitors for both C1 and C2 and would recommend using a lower range value on C1 for easier adjustment on 40m. Stainless steel hardware and rubber washers were used for the radial and radiator connections through the weatherproof box. A standard SO239 was mounted for the coaxial connection.
Due to 40m being an end fed 1/2 wave vertical dipole on this system, there is the potential for having RF on the shield of the coaxial feedline and radiating as part of the antenna. Several turns of coax were wound on a large diameter plastic tube creating a RF choke at the feedpoint to help reduce this effect.

Using EZNEC to model the antenna, it has good omnidirectional pattern on both bands with a takeoff angle of 24˚ on 80m and 19˚ on 40m. Since having it up for a few weeks now, on the air test have confirmed these patterns. I cannot hardly hear anyone within 400 miles with it on 40m but works well for long range contacts requiring lower takeoff angles. On 80m it seems to hear in a little closer than on 40m, but works good long range as well. I have made many contacts into Europe with it so far.

Below is a scan using the AIM4170 Analyzer. Click the image for a larger resolution. A portion of the video created about this antenna covers the analysis.

Hopefully this will help others looking for a good 80m/40m antenna. I have enjoyed this antenna project and it works very well.

73 and hope to see you on the lower bands!


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