Topband: 160M Short Verticals

[John Kaufmann]

TopBanders,

Here's another solution for improving the efficiency of short verticals that doesn't seem to be widely known.  Add a second (or even a third) identical short vertical in very close proximity to the first and feed them all in phase.  The phased system radiates as a single vertical with improved efficiency.  I believe this technique is documented somewhere in ON4UN's book.  The mutual coupling between the close spaced verticals drives the radiation resistance up, which enhances efficiency.

I've been doing this for years on 160 with three 60 foot verticals (actually my 80 meter vertical system) spaced 35 feet apart.  When fed in-phase, the feedpoint radiation resistance at each vertical is around 18 ohms without a top hat.  A single 60 foot vertical system has a radiation resistance of around one third of that.  When the total system resistive loss (ground loss plus other component losses) is high (much bigger than 5-6 ohms), the efficiency of the three vertical system would be improved as much as a factor of three (5 dB) over a single vertical.  For two in-phase verticals, the improvement would be around 3 dB.  When the system loss starts out low and the single vertical efficiency is pretty good, there is obviously less to be gained, but that's also true when other loading schemes are used with short verticals.   

Without a top hat, the in-phase short verticals do exhibit capacitive reactance at the feedpoint that must be cancelled with a loading coil.  In my case the coil is around 25 microH.  

The spacing is not real critical.  I use 35 feet because that's how my existing 80 meter verticals were already configured, but you can probably go much closer and still get the same benefit.  In fact, moving the verticals further apart would eventually reduce the benefit, so closer is probably better.  This can be modeled easily.

Some of you might be wondering why not phase the short verticals instead for directionality and hence gain.  In practice you will find that with any realistic losses, it's very hard to realize the theoretical transmit gain with very short verticals, although you can achieve good (lossy) directionality for receiving.  It's better to work on improving efficiency by reducing the effect of loss in the first place.

73, John W1FV