An Itsy Bitsy Teeny Weeny Upside Down HF Whip

If the title of this post has you thinking about yellow polka dot bikinis we are probably in the same age range. But today’s discussion isn’t about ladies’ swimwear, its actually about HF antennas.

Something has always troubled my mind when I think about very short whip antennas. Take the Elecraft AX1 for example – or going back a couple of decades – the Canadian designed Miracle Whip antenna. These are both inductively loaded short whip antennas. The whips on these products are only around 4ft long but they still require a decent counterpoise to work properly.

Looking at a short whip antenna we see, for example, a 4ft whip sitting on top of some form of inductor, connected directly to a transceiver. The antenna base is at bench height. But, and here’s the rub, the counterpoise (usually a quarter wave long) is going to be much longer than the antenna is high.

Doesn’t that seem incongruous to you? It certainly does to me. But what if we were to take that antenna and turn it upside down? Now we have a full length quarter wave radiating element and a short tunable counterpoise.

Take a look at geometry A and geometry B and you will see what I mean. I am advocating for geometry B and I will explain why.

Will it work? I decided to give it a try

The first thing to consider is how to connect the feedline. If this were a dipole it wouldn’t matter at all. One half of a dipole connects to the coax center conductor while the other half connects to the coax braid. But an antenna like the AX1 or Miracle Whip is an electrically equivalent quarter wave vertical whip worked against a counterpoise. The whip connects to the coax center conductor and the counterpoise connects to the coax braid.

If we simply turn that arrangement upside down, the whip – for which I substituted 4 feet of wire (to allow it to be suspended as a sloping counterpoise) – will still be connected to the coax center conductor. Most of our transmitted energy would be lost due to ground absorption.

So, instead, a means was devised to reverse the coax connections. Our initial counterpoise wire (now the radiating element) has been replaced by a 17ft whip (full length quarter wave on 20m) and is now connected to the coax center conductor.

The image above illustrates how it goes together. This new “upside down” arrangement changes the antenna from an incongruous compromised radiating element with a full length counterpoise into a full length radiating element with a compromised counterpoise. Does the radio care about the compromised counterpoise?

My theory is the radio doesn’t see a counterpoise wire – it sees a combined RLC (Resistance, Inductance, Capacitance). Most of the radiation happens on the radiating element so isn’t that where we should have the best efficiency?

Compromised counterpoises are actually quite common. Many hams use a quarter wave whip on a mag mount on their vehicle. Is the vehicle a perfect counterpoise? Maybe; my truck is a pretty good counterpoise on 20m, but too short for 40m. Does a vehicle provide a good capacitive path to ground instead? I doubt it, but it might be an interesting experiment to measure the capacitance between a vehicle chassis and ground.

Well, Did it Work?

Yes, it did work quite well in fact. I only tested the upside-downer on 20m, mainly because I didn’t have a whip for any other band. Maybe another time I will try hanging a vertical wire for other bands. I got an SWR of 1.1:1 and – just as an experiment – I changed the coil setting by sliding the tap point away from the best SWR point by a one or two coil turns each way. The SWR went up as soon as the setting moved away from the optimum point. The “Q” was quite high, meaning the tuning was fairly sharp, so I am calling it a success.

What is the Practical Use of an Upside Down Antenna?

In a previous blog post several months ago I recounted the tale of an inquisitive bypasser who stopped to observe what I was doing while I was activating a local park. My 17ft whip was mounted on a picnic table with four long radials spread out around the table. When advised to be careful of the wires on the ground she performed a little dance to avoid stepping on them.

I was concerned the incident could have resulted in a complaint to park officials (it didn’t) so I resolved to find ways to reduce my footprint when doing future activations in a public space. It was an important reminder to do everything possible to preserve our reputation as responsible operators.

Elecraft’s Excellent AX1

I have to emphasize that I have never owned an Elecraft AX1. I have read many excellent reports on its performance and no criticism of such a fine product is intended. On the other hand, I have owned a Miracle Whip. It too was a fine product and, in fact, was slightly more versatile than the AX1 in that it could be tuned to any band from 80m all the way up to 2m (the original product had a 57-inch whip and the inductance could be bypassed for 2m). I rebuilt my Miracle Whip by discarding the original whip, but keeping the superbly engineered rotary selectable inductor. The original whip was replaced with a 17ft telescoping whip to make the product much more efficient. I still use the modified Miracle Whip from time to time as a convenient antenna for QRP use.

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