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10M Squalo Antenna - WA1GPO

posted Aug 4, 2020, 10:20 AM by L Gray   [ updated Aug 4, 2020, 10:21 AM ]
New Project - 10m Squalo Antenna:

In addition to the antenna construction, I was also using a new device to characterize the antenna--a NanoVNA-F, Vector Network Analyzer. This was a bit of a learning curve for me. I started by monitoring Return Loss, SWR, Resistance and Reactance; ultimately I replaced the Reactance trace with a Smith Chart. This allows you to visualize the adjustments in real time. I remember being introduced to Smith Charts in one of my transmission line classes many years ago in college. I think I’ve forgotten more than I knew. Kind of exciting to revisit the theory after so many (many) years! 

On to the antenna ....

The 10m Squalo is an extension of the previously documented 6 and 2 meter Squalo antennas. Parts were obtained from a pair of lawn chairs and some scrap, salvaged from my stockpile in the back yard. A single length of 7/8 OD x 72 inch tubing can be purchased from McMaster-Carr for a reasonable price if you don’t have spare stock available to make the sides.

I tested two versions. The first version had a 4:1 step up transformer with a ferrite RF choke in a plastic box. The second version used a conventional gamma match. Bandwidth with both feed systems is about 700 KHz. The dimensions vary slightly as the wire used to connect the transformer feed effectively add to the element length.
( check out the pictures & comments below)

So, which version to build? 

The Gamma match requires more metal work, but is tune-able and slightly more efficient. It also requires more patience to tune!

The transformer version requires the matching box and some core winding in addition to the box construction. It is only slightly more lossy in the core.

Food for thought. Some details may be missing, contact WA1GPO for more information.
This is a nice antenna for casual operating or for our FARA round table gathering daily on 10m - 28411 @  11AM.  Join us and say hello--just an informal gathering.

The transformer version, with a temporary boom, dimensions are 45 x 46.5 inches ‘square’.
Matching box - ferrite choke on top & 4:1 Balun on the bottom.
VNA representation of a sweep from 25-30Mhz., resonance is at 28.4Mhz. This is the transformer version.
Gamma match version, size is slightly larger - 46.5x52 inches. Gamma rod is 3/8 x 24 inches spaced 2.5 inches C-C from the element. The gamma match element is fabricated from a piece of RG-8 coaxial cable. The shield is removed and the center conductor with the polyethylene dielectric is inserted into the 3/8 gamma rod to form the variable capacitor. The conductor extends 2.75 inches beyond the rod at the feed point. The rod is shorted to the element at about 17 inches.

The coaxial feed point can be fabricated from an SO-239 and some scrap aluminum. I also incorporated a coaxial RF choke into the feed, 17 turns of RG-8X wound on the 1.25 inch O.D. PVC pipe, again, what I had available. Element contacts are SS sheet metal screws and the elements are fastened to the PVC T's with aluminum pop rivets.
Final set-up in my shop. Take note of the stock and supplies in the background.
Final tuning of the Gamma match version. Note the SWR and the Z. Return loss is -35dB. Think of it this way ... RL is the highest at the frequency where most if the applied power is radiated (not reflected). Note also the spiral on the Smith Chart. The center of the chart is normalized at 50 +/- j0, that is 50 ohms resistive ... right where we are.