Sky Hook-II

It's a Marconi! No, wait. It's an end fed Hertz!

I designed Sky Hook-I for 40 through 15 meters. It also works pretty well on 80 and even 160 meters but at only 27.5 feet tall it's not ideal. Especially for chewing the rag with locals where an NVIS antenna would be much more appropriate.

So, I decided to put up some wire.

I have a couple of Pondorosa Pine trees about 150 feet apart. They're not very tall but there is a slight ravine between them and I thought "Hey, what the heck? They'll have to do."

So, I put up an Inverted-L and laid out 8 fifty foot radials. And that very night worked ON4UN on 160! (I mostly credit that QSO to ON4UN's fabulous antenna farm in Belgium. Nonetheless, thanks for the Q, John!)

Later, while reading an old antenna book I realized that my Inverted-L, being a quarter wave and worked against an RF ground was known as a Marconi Antenna. I guess if it was not cut to resonance it would be considered an Inverted-L. I like that it's a Marconi Antenna. It's good to honor our ancestors. Marconi was probably a pretty cool guy. I wish he would drop by the shack for a cup of java and see this stuff.

Setting aside, for the moment, historical references to the pioneers of radio I then decided to come up with a method to also use the antenna on 80 meters.

Stuck in my mind was a tidbit from an old 1970's issue of QST [PDF - members only]. It was a method (developed by Wes Hayward, W7ZOI) of devising an L-Network for dual band operation. I always wanted to try it out and this looked like the perfect opportunity!

But, first I needed to know the approximate impedance of the wire on 3.5 mHz. My antenna analyzer was not up to the task so I got ahold of a friend (David McGinnis, K7UXO) who lives and breathes antenna topics. David turned to modelling software to come up with an approximate value for me to use in further calculations.

The model indicated that on 80 meters my quarter wavelength 160 meter Marconi antenna would represent a load of 2867Ω +J117. About what one would expect from an end fed half wave – a very high resistance with inductive reactance. Hey, an end fed half wave?! Isn't that an end fed Hertz? I'll bet old man Hertz was also a pretty cool guy. It would be great to have him drop by the shack for an eye ball QSO with me and Marconi.

Calculating the values of an L-Network to match that complex impedance to 50Ω resistive was straightforward. An L-Network with a series reactance of 375 Ω (17 μH at 80 meters) and a shunt capacity of 120 picofarads would do the job.

But, the idea (as originated by Mr. Hayward in the QST article) here is to substitute the series inductance with a combination of an inductance and a capacitance that would be series resonant (0Ω reactance) at 1.8 mHz while simultaneously providing the required 375Ω inductive reactance at 3.5 mHz.

So, I worked up a little spreadsheet and quickly concluded that a capacitance of 325 pF and an inductance of 23 μH would work.

Frequency Picofarads Capacitive
Microhenries Inductive
Combined (series)
1.840 325 -266 23 266 0
3.550 325 -138 23 513 375

You can see that 23 μH in series with 325 pF provides a 0 Ω reactance at 1.840 mHz and 375 Ω reactance at 3.550 mHz. Perfect! So, I proceeded to assemble the Dual Band L Network as shown below:

Dual Band L-Network

L1 23 μH C1 325 pF C2 120 pF

Here is is a picture of the Dual Band L-Network. The coax N connector is on the left side, L1 is across the top, C1 (the series capacitor) is the vacuum variable and C2 (the shunt capacitor) is the air variable on the right. Finally, the output feed-through insulators are on the right side. The case was a water and air-tight transit case for an unknown type of military equipment.

Dual Band L-Network

Once assembled, I adjusted L1 to 23 μH and adjusted C1 for series resonance on 160 meters. I also adjusted C2 to 120 pF. I then installed the Dual Band L Network at the base of the antenna and connected an impedance bridge. L1 and C1 were adjusted for a good match on 160. C2 was, in turn, adjusted for a good match on 80 meters.

Here's what it looked like after a bit of "back and forth" tweaking:

SWR Chart

The question has come up (primarily from readers in Estonia): "what were the final values of the L-Network components?" Excellent question, thank you for asking!

I measured the antenna and found it to be resonant at 1750 KHz. That's a bit long for 160 Meter use but it worked great with the dual band L-Network concept. Being electrically long, the estimated resistance and reactance values I initially used were a bit off. That's why one uses variable components in such a project.

Final Values:    L1: 24.4 μH    C1: 273 pF    C2: 68.7 pF

I now have a dual band antenna covering the CW portion of the 160 and 80 meter bands with NO active switching or tuning. On 160 it's a Marconi. On 80 it's an end fed Hertz. It's automagical!

Many thanks to Mr. Marconi, Mr. Hertz, Mr. Hayward and Mr. McGinnis for their kindly assistance on this project!