QRO No-Tuner Multiband Dipole

Matt Roberts - matt-at-kk5jy-dot-net

Updated: 2014-04-18


A summary of the the construction details of a multiband dipole that can can operate at high power levels, and match its 50-ohm coax feedline without a tuner.

Previous Work

This antenna system is based largely on previous work done by amateurs for decades, using feedline segments of carefully-selected length for matching two feedlines of different characteristic impedance. Specifically, I built upon the work of W5DXP who built a similar system, and used relays to select the window line length desired for matching. His project used series transformer sections exclusively. The main change I made here was to use a combination of series and shunt segments.

Early Versions

A horizontal dipole is a very well-studied device. I won't belabor the details here. However, my current dipole is the result of a series of experiments in horizontal dipoles of varying heights, lengths, and orientations. Each revision was rather successful, and encouraged me to continue to improve the design. Each version was constructed from #12 stranded, THHN-insulated wire, fed with 300-ohm window line, and matched with an SGC-230 at the base of the window line, to attach to a coaxial feedline that led into the house.

The current design is a result of my desire to run high power (QRO) on this dipole, without having to invest in a QRO outdoor/remote tuner. Even the SGC-230 is quite pricey, and it has a 200W/80W SSB/CW power limit. Remote tuners that can handle higher power are unbelievably expensive. It's not hard to see why, because the RF voltages and currents inside the tuner's reactive elements can be blisteringly high at kW levels, even with mismatches lower than 10:1. The SGC-230's manual warns that RF voltages at the antenna terminals can reach several kV for high-Z antennas.

One of the last upgrades to the dipole before experimenting with high-power tuning was to change the 300-ohm window line to 600-ohm ladder line. The higher-Z open-wire lines experience less loss overall, which means that they can operate at higher SWR than the lower-Z lines for a given loss value.

Here is the result, a dipole with slightly elevated center support, 45' overall length, 35' high in the center.

Dipole Center   Ladder Line

The 45' length was chosen to accomodate frequencies 30m and above, fit well inside my small suburban back yard, and squeeze as much gain as possible from the antenna. At the same time, the antenna was kept short enough so that the main lobes at the highest frequency of interest (10m) were still normal (perpendicular) to the wire axis itself. The 45' length was the longest overall antenna that would meet these constraints.

A Little Planning

Any simple multiband dipole (i.e., those that employe a single conductor, and no traps) will offer an impedance that is widely different from 50-ohms, for most of the desired frequencies of operation. Some multiband dipoles are deliberately designed to offer a large mismatch on all of the desired frequencies of operation, in order to optimize certain bands, or to minimize the SWR on the largest number of bands. In any event, the the multiband dipole is not suitable for connection to coaxial cable, if low losses are desired.

The advantage to using a remote tuner between the coaxial feedline and the window line is that the high-SWR operation is limited to the section of window line, where the additional SWR-induced losses are much lower than they would be in an equivalent length of coaxial cable. Inserting a tuner at this point allows the tuning elements to be placed at ground level, which is far easier than trying to mount it at the top of a tower or mast. At the same time, the coaxial cable runs at very low SWR, keeping the feedline losses low.

So the most desirable path forward is to find a way to perform high-power, multiband impedance matching between the coax and the window line. This maximizes the actual power delivered to the antenna on every desired band of operation.

A Little Theory

It is well-known that two feedlines of very different characteristic impedance can be matched to each other by connecting them together with a segment length of feedline whose characteristic impedance is somewhere between the first two. Since the dipole was fed with 600-ohm line, and the coaxial cable from the transmitter was 50-ohm line, I chose to match them with 300-ohm window line. The equations for this are outlined in the Antenna Book. Since the 600-ohm line is attached to an antenna that rarely resonates at 600 ohms, the impedance at the match point is also unpredictable. So an additional step was taken to aid in the tuning process.

In addition to inserting a length of 300-ohm line between the two feeds to serve as the "transformer" segment, I also attached a length of 300-ohm line to the antenna side of the matching line. This open stub adds a small amount of additional capacitance on the antenna side, to help bring the match to 50 ohms. For operation on all frequencies above the half-wave resonant frequency of the dipole, this is sufficient to achieve a match very close to 1:1. From a matching theory standpoint, the tuning arrangement is somewhat similar to a pi or L network, where the stub is the antenna-side or "output" capacitor, and the series "transformer" section provides the transmitter-side or "input" capacitance and the inductor. In this case, the input C and L elements are distributed continuously throughout the transformer feedline section.

The SWR on the 600-ohm ladder line is still probably very high for any given frequency, but the match provided by the 300-ohm segments allows nearly 100% of the power provided by the coax to be transferred to the ladder line. Since the ladder line's overall segment loss is quite low in the HF frequency range, the additional loss incurred by all the reflections due to the high SWR is minimized. By providing a 50-ohm match point at the load end of the coax cable, the high-SWR region is confined to the ladder line, and the few feet of 300-ohm transformer segments.

The No-Tuner "Tuner"

The length of the segments needed to match the dipole depends on the frequency of operation. It should be possible to determine the lengths needed mathematically, but instead, I used EZ-NEC+ to simulate the feedlines, and experimentally determine the lengths required for each band of interest. Of course, these are approximations, so some fine-tuning was required when the stubs were actually installed.

Rather than cut stubs of specific lengths, based on the models, I used another thought from W5TXF's site, and cut several segments of varying lengths, starting with 6", and doubling each piece's length, for 1', 2', 4' and 8' lengths, making a "kit" that can be used to assemble any length from 6" to 16' in 6" increments.

To connect the segments end-to-end, I used PowerPole connectors. In a very convenient coincidence, the spacing between the wires of a 300-ohm line is almost exactly the same as that between 45-Amp PowerPole pins. So these make excellent quick-release connectors for 300-ohm line.



To connect the stubs to the antenna, a panel was assembled to bring it all together. The panel was built using 1/4" hardware, installed onto a switch plate. The switch plate is a thick, plastic, industrial-grade plate, and the 1/4" hardware is spaced to match the 600-ohm line. To this, two short lengths of wire connect the 1/4" bolts to the two sets of PowerPoles, one for the transformer section, and one for the open stub.

A resistor of several megaohms was placed across the 600-ohm terminals to drain static buildup from the antenna. This is far too much resistance to significantly alter the impedance of the antenna, but it keeps high static voltages from building up across the antenna leads. Since one side of the ladder line is DC-grounded through the coaxial cable (which is in turn grounded at the house entrance panel), this arrangement will drain all static build-up to ground before entering the house.

One of the power-pole connectors is used for the series section, and one is used for the shunt stub. An adapter converts 300-ohm cable to 50-ohm cable, and serves as the last few inches of the series piece. A choke is used to prevent line imbalance from causing RF radiation or pickup from the coaxial shield.

Match Panel   RF Choke

Switching bands only requires replacing the stubs with those of the correct length. The existing PowerPoles are simply disconnected, and the correct lengths of window line are inserted. Starting with the lengths predicted by EZ-NEC+, I used a ZM-30 analyzer to fine-tune the lengths of both sections for each band of interest. Since a 45' dipole is electrically long on all bands at and shorter than 30m, the bandwidth was plenty wide. I found that 6" was more than sufficient resolution to find a 1:1 match point within all of the various CW, Data, and SSB subbands for these wavelengths.

The match panel shown above is located on my back porch. I pulled the 600-ohm line from the antenna to the porch, and buried the coaxial feeder from that point to the entry panel elsewhere on the back of the house. This way, I can change the matching sections without having to go out into the yard. If I add relays, as is the case with W5TXF's installation, I wouldn't even have to go outside.

The ZM-30 has a nice feature that allows it to sweep through a range of frequencies, and emit the results to a text file. That file can later be imported into a spreadsheet for charting and analysis. I did this with stubs of various lenghts, looking for the resonant points for each configuration. When compared to the SWR chart output of EZ-NEC+, this allowed me to see how the antenna model compared to the actual construction, and it helped to refine the EZ-NEC+ model.

Summing it Up

Using this system, the coax can be matched at exactly 50 ohms on any band from 30m through 10m. Since no lumped reactive components are used, the matching system can be used for any power level within legal limit. At the limit of my station, 500W, none of the components show any signs of heating, e.g., SWR drift at high duty-cycle, arcing in or around the PowerPoles, distortion of feedline plastics, etc.

In theory, moving to 450-ohm line would decrease the losses of the tuning elements even further. This would be an inexpensive experiment to try. I suspect the improvement would be negligible, since the tuning elements on all non-WARC bands are only a few feet long.

One last point of improvement would be to add relays to switch in and out the various segments. I have a plan for doing this using a Wi-Fi Arduino board, or similar kit computer. That will be a subject for another article.


W5DXP's No-Tuner Dipole
ARRL Antenna Book

Copyright (C) 2012-2014 by Matt Roberts, All Rights Reserved.