Recently I have revised the rod antenna for my handheld QRP SBB transceiver. The main objective was to simplify the matching circuit. As I’ve pointed out before in the antenna article, one of the major problems with shortened antennas is the low feed point impedance. The first version of the antenna matcher used a capacitor and a coil to form part of a PI-filter. This new circuit uses an autotransformer made of a linear coil.
The tap is at about 1/4th of the total windings which transforms the feed point impedance of about 10 ohms to the coxial cable with 50 ohms. A 5 meter wire acting as the second part of the dipole should be used to increase performance of the rod antenna. All dimension concerning antenna length have remained unaltered, please see respective article!
For my handheld QRP transceiver I have developed a rod antenna for outdoor use when cycling or hiking. In this article I’ll first give the reader some basic considerations on shortened antennas and afterwards the pracital consequences of these.
A full-sized vertical antenna usually has got a length of a quarter wavelength. Some special constructions of a 1/2, 5/8 wavelength and others are also commonly known. But for a rod antenna the basic construction is a 1/4 wavelength based circuit. Full-size 1/4 wavelength antennas that have the correct length (1/4 multiplied by 0.95 shortening factor) have got a feed point impedance of between 40 to 50 ohms. So they can be directly connected to a 50 ohm coaxial cable. The problem: For 14 MHz this antenna would have an overall length of about 5 meters. Not very appropriate for a handheld transceiver except you were Arnold Schwarzenegger.. 😉
When the mechanical length of an antenna is shorter than 1/4 of the wavelength that it is desired for, a mismatch can be obeserved. The antenna not longer is resonant to the operating frequency. An additional capacitive reactance appears. This must be compensated by an inductive reactance. Therefore shortened antennas have an integrated coil either at the bottom (bottom loading coil), anywhere in the middle (center loading coil) or top (top loading coil).
Another problem must be faced: Feed point impedance is usually much lower than for a full size antenna. Sometimes only between 10 and 15 ohms. Thus some sort of impedance matching circuit must be integrated, too,
Last, degree of effiency will also decrease. So, don’t expect too much from a shortened antenna!
To sum up the things that have been mentioned before: We have to face 2 general electrical problems with short antennas:
A correct loading coil must be found, and
proper impedance matching must be performed.
Into my antenna I integrated 2 loading coils. One at the bottom to serve as an impedance matcher and one slightly below the center to compensate capacitive reactance.
The antenna is mounted to a standard male BNC-connector.
Mechanical deimensions and coil data:
Bottom coil (L2):
L2, the bottom coil, is 50 turns of 1 mm diameter enameled wire wound on a 8mm diameter plastc tubing from the local hardware store. From the bottom end of L2 a 120 pF capacitor is lead to the ground potential of the BNC plug. This, together with L2 sets up low pass filter serving as an impedance matcher.
Edit: Another impedance matching circuit I’ve described here.
The plastic tubing used for L1 and L2 is hollow. Into this tubing a 6mm diameter aluminium rod fits in exactly. The length of the rod between the two coils is 45 cm. Following is another piece of the plastic tubing carriyng L1.
Center coil (L1): L1 is 45 windings of 0.6 mm diameter enameled wire.
The top rod of the antenna is a telescopic antenna with an overall length of 120 cm that can be bought on the internet. It’s outer diameter is also 6 mm, so it also fits into the plastic rod.
Here are the pictures to make clearer how the antenna is constructed:
To increase the usually low degree of efficiency of such an antenna I have manufactured a sort of “counterpoise” that represents the 2nd part of a dipole : A 5 meter long insulated cable with a large crocodile clip that is clipped to the ground potential of the metal BNC connector.
Edit: In the meanwhile I have used this rod antenna for 14 MHz / 20 meters several times during outdoor activities. What I’d never have believed: I could make lots of QSOs with it! Particularly if you are in a high place (e. g. on a look-out) you can work distances of about 1000 to 3000 kilometers right with 4 watts out of your hands. Provided the station you’re answering is strong. Then there is a realistic chance that he might hear you with reasonable signal strengh.
I have really been satisfied with my last QRP SSB rig. It performs very fine. But I wanted a transceiver still a little bit smaller. And it should be easier to set up the rig if you are outside to make quick QSOs. This and the fact that I came across a larger bunch of 9.832 MHz crystals which I thought could be an ideal basis for a ladder filter made me plan an even more compact rig for 20 m compared to the last one. Particularly for portable operation on holiday or when I am outdoor with my bicycle or hiking, I wanted a self-containing transceiver with on-board battery. So here it is…
Monoband SSB-Transceiver for 20 Meters/14 MHz.
Output: 4 to 5 Watts PEP
Frequency generation: DDS with AD9835 and ATMega8, LCD 2×8 Characters.
Transmitter: SSB Modulator for USB or LSB: NE602/SA612, 4-Pole-Ladder-Filter, TX-Mixer, NE602/SA612, Power amplifer: 3 stages, push-pull-final with 2 x 2SC2078.
Receiver: Singe conversion superhet, RF preamp with Dual-Gate MOSFET, RX-Mixer with NE602/SA612, 4-Pole-Ladder-Filter, Passive product detector, AF preamp, AF final amp with LM386. (See improved AGC circuit here!)
Built-in battery pack, also connectable to external power supply, 10-LED-bargraph-display for S and RF strength readout.
The size is about that of those older CB handhelds from the late 70s. It is abt. 18 cm long, 7 cm wide and 4.5 cm high. But don’ ask me for weight. 😉
The transceiver has, as mentioned before, a power output of 4 to 5 watts rf pep which I found sufficient for making contacts worldwide. Transmit power mainly depends on the respective power supply you use. The radio can be run either on an integrated 12 V dc rechargeable battery (1Ah, composed of 10 AAA cells) or by connecting an external dc power supply of up to 14 volts. A three-position switch allows the user to select either internal or external power supply or completely switch the rig off.
Thus the little radio is very versatile for all kinds of portable operation. The antenna is connected via a standard bnc connector. I also designed a rod antenna that you can use if there is no possibility to erect a larger aerial. I have to admit that having a qso with the small handheld antenna ist a pretty ambitious. 😉 But in recent “The King of Spain”-Contest I could work with the rod antenna from a high place over a dozen stations.
With my delta loop the rig is absolutely amazing. During the recent weeks I worked (among others) the following prefixes:
And this all was done, except from the contact to P40FN, with 4 to 5 watts pep. Only for the QSO to Aruba I had to use the 60 watt linear amplifier. The pile-up was too hard. 😉
Basic design ideas:
To make the rig not too bulky I used a “sandwich construction” in an aluminium frame. The radio mainly consists of three layers:
The RF and AF unit (mainboard)
The battery-, AGC/Meter- and switching unit
The display and push-button unit
The RF unit:
On board here you can find the DDS-VFO, the whole receiver and transmitter circuits (including power amp), SSB modulator and demodulator.
The battery and AGC/Meter and switching unit:
This board is equipped with a set of 10 rechargeable batteries (1.2 volts each), the relay for transmit/receive switching and the LED-S/RF-meter circuit (LM3915) plus the AGC device. Above this section there is another layer which keeps the 2×8 line LCD, the up/down control switches for tuning (there is no rotator tuning knob), a button for setting the tuning frequency step, selecting the VFO (4 VFOs and 2 split VFOs are software defined) etc. It is integrated in an extra housing mounted on top of the cabinet an connected via some cables. The controls are simple push-buttons. With one of these the microcontroller can be set into sleepmode to reduce the radio’s noise down to a minimum. The 1 inch loudspeaker is also mounted in here.
The circuitry itself is standard QRP design with the “usual suspects”. See the schematic to learn more:
The front panel looks like this:
Details are to be discussed in my next posting on this blog. Thanks for watching! 73 de Peter (DK7IH)
In my junk box I found 2 transistors MRF 455, each capable of delivering 60 watts of rf power. I quickly made up my mind that I wanted to construct a useful circuit for these two high power rf transistors. A linear amp for my 5 watts 14 MHz QRP SSB transceiver was the goal I had to achieve in order to get a little more power for the tiny transceiver.
And, in addition, I also have had an amplifier strip of a blasted Atlas 215 transceiver for about 20 years that was destroyed because of using false polarity power supply. The only thing useful that survided this massive destruction were the broadband transformers for input and output. So why not trying the reconstruction of the famous PC500-board of the Atlas amp as a QRP linear power amplifier that can be used when band conditions are low and a few watts of extra power are needed?
So, here is the circuit of the PA:
As you can see from the picture, the rf transformers T1 and T2 are made of big ferrite beads. The inlays are brass or copper tubes serving as rf leads representing secondary (T1) or primary (T2) winding. The tubes on one side are connected by a large solder bride serving as the tap of the winding. The primary of T1 are 3 turns of thick insulated wire, the secondary of T2 are 2 turns of the same material.
I built the amplifier on a copper plated pc board. The single solder pads are handmade using an electric mini drilling machine device carrying a small cutoff wheel. The remaining copper gives a suitable ground plane for the PA circuit preventing instabilities. More complex circuitry like the adjustment circuit for the idle current was conventionally made on a veroboard. Idle current should be set to 100 mA.
L1 and L2 in the final low pass filter are 9 resp. 10 turns of 1mm diameter enameled wire. Coil diameter is 8 mm. C16 is 68 pF, C14 is 120 pF and C15 is 100 pF. Each capacitor must be able to withstand 120 V at least! I recomend testing your own values!
What I didn’t point out in the schematic above is the relay that controls signal flow (it’s really trivial to wire that) and the hf vox which can be found on the internet in many variations.
The bottom part of the enclosure I constructed is made of 4 mm (!) aluminum plate. Due to the fact that I intend to use the PA for SSB only I did not use an extra heat sink. The cooling by the large and thick aluminum plate is sufficient for ssb operation. The whole body therefore became very slim.
Driven by my 5 watt QRP SSB transceiver for 14 MHz the linear amplifier puts out about 60 watts in the peak with perfect signal quality. So I finally got a good new workplace for my surplus MRF 455s and the 2 rf transformers from a destroyed ATLAS transceiver I can use when “life’s too short for QRP”. 😉