Friday, November 18, 2016

"Broadbanded" 40 meter vertical antenna

Can you spot the antenna?  Of course YOU can, but it's proven relatively invisible to the typical non-radio cuckoo person thanks to a can of camouflage paint (Rust-Oleum "Deep Forest") from Lowe's.

There's nothing remarkable about the antenna itself, it's just 40' worth of aluminum sections mounted to a treated 4x4.  The sections telescope into one another, allowing the length of the antenna to be adjusted to bring it into resonance.  Very simple, but, no vertical will perform worth beans without a decent ground system.  Again, mine is nothing special: just a bunch of radials, between 30 and 100' long, buried just under the sod.  It's been several years since I installed this antenna and I don't remember exactly how many, but it's somewhere around 20...  I had a bunch of partial spools of wire and just kept burying them until I ran out :-)

Here's the cool part: On the lower frequencies, it's tricky to make a single antenna present a decent match across the entire band without some kind of tuner. A long time ago, someone showed me a simple way to "stretch" the bandwidth of an antenna by adding an electrical 1/4 wave section of 75 Ohm line to the base. So, at the feedpoint of this antenna is an electrical 1/4 wave section of RG-11, which connects to an electrical 1/2 wave section of RG-8 that feeds a bandpass filter located in a weatherproof box at the base of my tower, where it joins the section of RG-8 that runs through the underground conduits (about 150') and into the shack.

A brief description of how I understand it to work is: At resonance, the inductance (L) and capacitance (C) of the antenna and feed system are equal.  As you tune in frequency away from resonance, the L and C of the antenna and feed system change in opposite directions, countering each other; As the antenna "goes capacitive", the feed system "goes inductive".  Obviously, there are limits, otherwise the thing would look like 50 Ohms across the spectrum, but in this case, it's enough to provide a decent SWR across the band.

It's a simple trick that doesn't add appreciable cost or complexity to the system, give it a try!

Base of antenna - lag bolted to 4x4

View looking up from eye leve

2 comments:

  1. Donald A. King de KD8SFZNovember 19, 2016 at 12:10 AM

    Nice job very informative thank you

    ReplyDelete
    Replies
    1. Thanks, Donald! I enjoy sharing this stuff with "the Brotherhood"

      73 - Steve

      Delete