Efficient Half Size Dipole for 40 meters

In my continuing saga of antenna experiments, I have designed  a 30 (a little less than 1/2 normal size) foot long 40 meter dipole that is a solid performer. The goal was to make it short enough that it would work well in portable applications as a vertical or sloper hung from a tree.  It is center fed with one continuous loading coil tapped at the center two turns to a SO-239 type jack for coax. A balun is not necessary because of  the tapped connection to the loading coil. Along with the loading coil, there are two cylindrical capacitive hats which  replace about six  feet of wire each. The hats improve current distribution, bandwidth and efficiency,  allowing for a smaller loading coil. The hats are lightweight and flatten out for very easy transport. The antenna uses carabiners and inline connectors to allow quick connect and disconnect for setup and removal.

Link To STL Files


Antenna mounted as a sloper


The Loading Coil and Hat frames were printed on my 3D printer using a low RF loss plastic material – High Impact Polystyrene(HIPS). Here is a link to the STL files which can be used to print these forms. An STL file is the standard format used by almost all 3D printers. If you do not have a 3D printer – there are online services available to 3D print the files as well as some print shops and office stores. Other plastics could be used such as ABS but use HIPS if you can. Because the antenna is relatively small you can mount it vertically or nearly vertical with the center relatively high above ground for better efficiency so besides being small, the antenna has some benefits with respect to ground losses and radiation angle (depending on how you mount it).

Loading Coil

Coil has 11 full turns(plus half a turn) to center from each side(3 inch diameter and 3/16ths turn spacing). It can use 12 gauge or smaller wire. The mounting holes can be tapped for 6/32 or 8/32 screws for the coil terminal and connector flange. Total coil inductance between 16-17 uH

IMG_20151031_170006399 IMG_20151031_165858898

Capacitive Hat

The hats are 6 inches in diameter with the wire soldered together in the center and then pinned with a 8/32 screw and washer.


Antenna adjusted to resonance for 40 meter CW


Ultra Simple 8 pole Low Pass CW filter

I wanted to add more audio filtering to my 40 meter CW transceiver https://circuitsaladdotcom.files.wordpress.com/2015/08/qrpxcvr2.jpg  but didn’t want to put much effort into it, so I used a MAX7401, 8 pole switch cap filter IC. This IC requires just a few discretes and has an extremely low passband ripple and group delay, as it is a Bessel configuration. I have used this IC before in some of my guitar effects pedals. It has worked very well in my other designs. The knee of the filter is adjustable via a capacitor(C4). To integrate into my receiver, I simply connected the input and output of the filter across the input capacitor(C27 removed) connections to the final audio amp. It may be desirable to put a single RC low pass filter stage in series with the output of the switch cap filter to remove clk artifacts on the output.(clk is 100X greater than the roll off frequency). Performance is very good with clear tone and no ringing.

Below, I have a video demo of the XCVR utilizing the filter. This version of the XCVR is tuned via a POT connected to the microcontroller A2D converter instead of a rotary encoder. It also has a push button to shift in 5Khz increments. When the button is depressed for longer, the frequency in KHz is sounded in morse code. Only the KHz is sounded, so for 7.100 Mhz for example, only 100 is sounded in morse code. In the video you can also see my cool 3D printed Code Key. It uses rare earth magnets instead of a spring for key action. It’s a really delightful bug.

Picture of filter Daughter Board connected to XCVR


Filter Schematic

Simple CW filter

Video of XCVR using the Filter

Improved Power Toggle Latching Circuit

Awhile ago, I posted an example of a Power toggle circuit using a handful of components that I  use all the time to allow a momentary switch to control Power On/Off function.


The circuit works well but can require a series diode with the output to prevent  stored charge or magnetic energy from re-energizing the lating function such that it won’t turn off. The diode drop may be undesirable, so I modified the the circuit  as show  below.

Alternate Power Toggle

The addition of R7 and C8 eliminates this problem in every circuit I have tried and is sure-fire in reliability. Normally, C8 can be omitted, but I added for one application with very strong RF fields near by.  As before, the circuit draws no current when off and can toggle within a second or so.

Of course you might ask why not just use a toggle switch?  Well of course you can but you can get momentary switches cheaper, they can be much smaller and generally I find switch bats ( toggle levers) to be un- aesthetic and sometimes cumbersome. So now I usually do something like this.

More Spiral Experiments

I have come to the conclusion that my large(1 meter diameter) spiral loaded vertical is really an asymmetric dipole with the spiral being the other radiating element.  For fun I made a much physically smaller spiral and attempted to use that.

New Small Spiral


I found it was extremely difficult to tune and narrow band as well. It did work but the receive  level went down slightly  so I have concluded it is not  as efficient as the large spiral. Feedpoint impedance measurements were better matched(higher R) than the other spiral but this is surely IR loses in the dense coil.

My conclusion is a moderately sized spiral of a few turns(> 1/20 wavelength) is very effective as its area makes it a respectable radiator. If the spiral coil becomes small the approach doesn’t work so well.  My large spiral is a solid performer and it is my opinion that it is preferable to a modest radial array.

Vertical with Tuned Spiral Counterpoise – Updates

So I have been using the my center loaded 40 meter vertical with a spiral resonant counterpoise for a couple of weeks and can say it is performing as well (perhaps a little better) than with my 24 ground radials. I did some simple field strength measurements with a spectrum analyzer and pickup coil; at about five different positions and at various frequencies in the 40 meter band. I did this with the spiral version and with 24 ground radials. Comparatively, they were within a dB of each other. The spiral was about 1 dB better in three of the positions and both antennas were about the same in the other locations.  However, this was only the case when the spiral was adjusted properly(less than 3:1 SWR).

I improved my loading coil by 3D printing a low density coil form and added a capacitive hat to improve the performance of the radiator as much as possible

IMG_20150911_195109288_HDR IMG_20150911_195149153

The 2:1 SWR bandwidth is small (50KHz) and if you move it around you need to retune it -so its more critical to adjust and less forgiving than when radials are used. The footprint is much smaller though and I have easily set it up in my driveway, on my deck, and over grass (about 5 feet off the ground). Generally, I am achieving the same range, signal reports and number of contacts as before. Using the spiral decreased the bandwidth of the antenna and lowered antenna impedance, while using the same exact 1/4 wave element I was using previously with radials.

I created a simple L matching network for the antenna which works beautifully to convert the 20 ohm Z to 50 ohm at the feed point. You connect the coax ground to the spiral for resonance, then simply connect the L network ground connection to that same coax connection point. If not optimal, you can slide the L network ground connection  along the spiral wire a few inches on either side of the coax ground connection to tune. The values for the L network are .5uH and 470 pF. You don’t need the network if you can tolerate the higher SWR but it feels good to see such a nice match!

L Network used to match spiral               

IMG_20150906_193630496 IMG_20150906_193730190_HDR

Update: 10/05/2015

So I have done some more experiments with field strength and also done some research and found this is a topology that is a a variant of a spiral antenna. It has been coined the “spiralpole. Essentially I have created a vertical dipole where one half is the spiral element and the spiral is actually radiating significantly. This is particularly true because my spiral area is large and the number of turns is low. I had a feeling this was the case( the spiral was radiating). So the counterpoise is in fact now a radiating element. It seems to work. I plan on making a much smaller spiral coil and see what happens.

Short 40 meter Vertical with Tuned Spiral Counterpoise.

I took my 20 foot short 40 meter vertical and tried using a spiral tuned counterpoise instead of my 24  radials. The counterpoise is off the ground only about 5 feet so the antenna is still ground mounted.  Before I describe the details of the design. I want to discuss performance. With ground mounted radials I was getting a little over 50% efficiency. This is derived from a predicted feedpoint impedance of 18 ohms, but measured impedance being 31 ohms(13 ohms of losses). Using the spiral counterpoise; the feedpoint impedance is about 21 ohms – which implies >  80% efficiency. The only draw back is the tuning is a critical with about 6 inches difference between un-usable and perfectly aligned. As a practical approach, I made the spiral a little long and then I adjusted my coax ground connection point- to tune the antenna. I have made no effort to match perfectly to 50 ohms and the SWR is 2:1- over about 100 KHz. Tuning need only be adjusted when using a different portion of the band or moving physical position is some significant manner(placing lower to the ground for example). Regardless, there will only be about 6-8 inches of adjustment ever needed to tune the antenna. A  L network or broadband transformer may be desired, to match to 50 ohms. Adjusting the feed point up the antenna a little, should also work to get a better match. The counterpoise appears to work very well and I can recommend it as an alternative to radials. I am assuming the radiation pattern is the same but I have not evaluated this. Of course making contacts is not a  good measure of antenna performance but I have made  contacts using the antenna in this manner -so it certainly works. Based on my measurements, it has less loss using the spiral counterpoise vs ground radials.

Short Vertical  Tuned Antenna Impedance with Spiral counterpoise

20 ohm Z with almost 0 reactance (-1.6 deg)


Tuning the antenna element to resonance first (using a dinky ground plane…or a good one!) is prudent, because it is very tricky to simultaneously tuned the antenna and the counterpoise. Once the radiating element  is tuned correctly, tuning the counterpoise is  straightforward. I just a have an alligator clip to shift the ground connection point on the counterpoise.

Completed Counterpoise installed and tuned


Complete Antenna


The counterpoise is  a large square coil made on a 1/2 inch PVC frame. Four legs of 1/2 inch PVC (each leg is about 21 inches long) are connected by means of a four-way coupling which is pinned within a drilled 2 inch coupling. A piece of 1-1/4  inch PVC pipe is used to hold the fishing pole mast on the top and provide a means to mount on the ground. 2″ to 1- 1/4″ reducers are glued into the 2″ coupling. The entire coil is 5 and 3/4 turns spaced at 3 inches between turns. The turns start at 5 inches from the center. I measured  3 inch spacings on one leg and then did the same on the other three other legs. On the other legs however, I shifted the 3 inch point forward 3/4 of an inch on each leg with respect to the previous leg in the the winding order. This way the wire gradually shifts outward without abrupt turn hopping. I used #12 bare wire -you will need at least 40 feet of wire to make the coil.

four way PVC coupling and drilled 2 inch coupling

(the four way has to be trimmed 1/8″ on the ends to fit)


Coupling held in place with 1/2 inch stubs


1 1/4″ reducers are press fit and glued into the 2″ coupling