Have a really cramped area available and want to operate on 40 meters? Build yourself a “postage stamp” 40 meter wire dipole antenna that fits in a space a little over 20’ wide and works reasonably well at low heights (20′). The antenna requires (2) 17 uH loading coils placed at the ends of the 10’ horizontal radiators. A 10’ radiator is attached to the lower end of each loading coil. Tuning is accomplished by adjusting the lengths of the lower radiators.
An EZNEC model of the antenna was created and evaluated. The modeled antenna exhibited acceptable characteristics on the 40 meter band and good characteristics on the 12 meter band. The predicted radiation pattern indicates the antenna can function as a local/regional antenna.
EZNEC Model Displays
My friend Barry (KI4PMI) and I built a postage stamp 40 meter dipole antenna. We used #14 AWG black THHN stranded wire for the radiators and loading coils.
Since the lengths of the antenna radiators are considerably shorter (20’) than the radiators of a typical horizontal 40 meter dipole (33’), loading coils are needed to compensate for the shorter wire lengths. How can the loading coil inductance (uH) be estimated? The online calculator at the link below is a good starting point. Add at least 3 uH to the value displayed to allow for factors not considered by the calculator. It is easier to remove turns of wire than it is to add turns after you start winding the coils.
Another approach to estimating the required inductance is to adjust the inductance of the loads in an EZNEC model until the best SWR curve is achieved. You may have to “tweak” the lower radiator lengths in addition to adjusting the inductance of the loads. A copy of the EZNEC model we used is available at the link below.
Note: You can download a free DEMO version of EZNEC 5 from the link below.
We used the EZNEC antenna model to estimate the inductance (17 uH) required for the coils. Using Wheeler’s coil formula, we estimated that 16 turns of wire closely wound on a 2” inside diameter form made of Schedule 40 PVC pipe would produce the required 17 uH.
Wheeler’s Coil Formula
We wound two coils and analyzed them with the AIM-4170C to insure they were closely matched in Q, resistance, and inductance.
Note: An online implementation of Wheeler’s coil formula is available at the link below. Be sure to set the units to match the dimensions you are using. Typically you will use inches. The wire diameter for #14 AWG is 0.0641″. Use 2.375” as the coil diameter. That is the outside diameter of 2” inside diameter Schedule 40 PVC pipe. Since the wire will be wound on the outside of the form, we use the outside diameter in calculations.
Tip: To retroactively improve the EZNEC model, you can add the measured R value from the coil analysis to the load parameter specifications and re-run the model. You may have to “tweak” the lower radiator lengths.
The completed antenna was hoisted it to a height of 22.5’ and analyzed with the AIM-4170C.
The measured SWR curve of the antenna is relatively close to the SWR curve predicted by the EZNEC model of the antenna.
As predicted, the antenna performed well locally and regionally on the 40 meter band. We were able to check in with ECARS (East Coast Amateur Radio Service; net control located in MD) on 7.255 MHz with a “59” signal report. The on-the-air test was conducted in Clayton, NC.
If you are interested in the 12 meter band, the antenna will also perform well on this band.