August | 2015 | Circuit Salad

I like to work QRP CW and exclusively use home brew portable equipment. I have struggled with the best antenna to use for 40 meters. Wire antennas are lightweight and usually efficient but often require tuning, a structure or tree to install, and often end up low to the ground with poor DX performance. A 1/4 wave vertical is a good choice but is 33 feet long and requires lots of radials for acceptable efficiency.

What I came up with is nothing new but I was able to verify good performance and efficiency.

I have constructed a center loaded 20 foot vertical using a 20 foot collapsible fishing pole as the mast. The loading coil is wound with 12 gauge bare copper on a couple of PVC 2 inch couplings joined together. The coil is about 12 uH in value. Final tweaking is accomplished by creating a tiny loop at the top of the antenna(2 inches of area) that can be varied in size(create a small amount of capacitive top loading). The radial system consists of 21 ten foot radials. Since they are short the footprint is small and setup is easy. Using more short radials is more efficient than using a few long ones(up to a point). Unless you are going to use 60 radials or more there is no reason to have them 1/4 wavelength long. This is because current density is greatest at the feed point and diminishes as the radials extend out. Roughly, 24 radials at 1/10 wavelength long have the same performance as the same number of radials at 1/4 wavelength long.

From my measurements with a network analyzer, the resonant feed point impedance is 30 ohms and calculated impedance with zero losses would be 18 ohms. So its on the high side of 50% efficient, which is good for a portable vertical.

I have made a couple of cross continent contacts with good results using 1 watt, so I am pleased with the ease of install, small footprint, portability and effective radiating.

Antenna stuck into my Yard

Close up of the loading coil

Short Radials at Base

This is my new qrp transceiver design. It uses the CS2000 clock generator circuit I use for a number of my receivers and a 74AC240 buffer as a push pull 1.5 watt final amplifier. The final unit uses my low power/low noise LCD display and is set to tune from 7.000 to 7.300 MHz.

https://circuitsalad.com/2014/07/31/very-low-power-3-digit-lcd-display-with-serial-control/

If one changes the the output networks (L matching and half wave low pass) and simply adjusts the software for tuning range, one can use any band desired. The inductors are air wound or made using scraps of ferrite taken from junk box inductors. Tuning is accomplished by means of a cheap mechanical rotary encoder. The Micro-controller provides the control signals for T/R switching, generates a sidetone and shifts the frequency 700Hz on TX. The transceiver supports full break-in keying. The input to the receiver is isolated during TX by means of a BSS123 mosfet used as a switch. A 2N7000 will work here as well. The receiver uses a leftover inverter stage as a RF amp. It works surprisingly well and also provides the bias for the switching mixer input. There is a 2nd order op amp low pass filter for CW. The coupling capacitor C30 is normally not needed for receive, but when the TX is keyed, there is DC imbalance on the output of the mixer switches – the differential audio amp will slam to the rails creating large key clicks on the side tone. C31 is required for stability when keying, without it, occasional parasitic oscillations can develop.

The circuit could be easily modified to use a VXO or VFO. One would simply have to modify the keying logic(connect key directly to TXRX signal and invert this signal for MUTE connection) One can use the remaining two 74HC04 inverters to generate a side tone and the the logic.

Update: Changed the band to 30m for fun. The only hardware changes were: L1 & L2 (.11 uH), C21 (800pF), L3 (.6uH), C22 & C23 (270pF). The 74AC240 works beautifully at 10 MHz – outputting the same 1.5 watts and even running a few percent more efficient. I must have nailed the matching network values.

Schematic(minor edit for key clicks 8/18/2015)