M0PLK Multiband Delta Antenna
By Arthur M0PLK (SQ2PLK)
A lot has been said and written about multiband antennas, which can do miracles for amateur radio operators. However, it is always the case that a multiband antenna is more or less a compromise solution. The same is true about the antenna I describe below, modelled after the 7PL two-element Delta Multibander.
The antenna proposed by me is a single-band Delta fed at the top of the vibrator element through a 450-600 ohm ladder and a transforming system.
The assumption made for this solution was that the antenna would be used in portable conditions, and that is why the design was characterized by simplicity of construction and short assembly time. After folding, the antenna can be placed in a standard bag for angle rods, as you can see on Fig. 1.
The materials include three telescopic masts made of fibreglass that can be bought in an Internet store, although stores for fishermen or gardeners can also offer similar materials. Two telescopic arms of 6 m each were mounted on an aluminium mounting plate, using power cable holders (at the right angle in respect of each other and at 135 degrees in respect of the lower section of the mast).
The mounting plate was used for mounting the whole structure to the mast (the total fibreglass mast height was 9 m, but only its lower sections were used), with the application of the same assembly method as in the case of the antenna arm mounting. As you can see on the picture, the same method was used to mount the transformer box.
Three guy cables of about 6 m each fix the whole structure vertically.
Please notice that the mounting plate is fixed on the last section of the mast which is not fitted to the telescope, but rather laid loosely on the lower element. That solution allows for easy rotation of the whole structure to use the antenna's directionality, with an additional control rope (Fig. 2).
The radiating part of the unit described here was supposed to be lightweight, and that is why I applied a thin loudspeaker cable for both Delta wire and feeding ladder.
In its main parameter, the antenna length is 17 m long, which allows for operation from 30 m up to the 6 m band. The cable mounting to the antenna arms was executed with an insulation tape. If you plan to use the antenna in the 6 m band, you should remember to keep the distance between the ladder wires larger than 6 cm. The ladder impedance and the ladder length do not affect the antenna's radiation properties significantly. The ladder length should be selected in the way to make it adequately tense (in the original unit, the length was about 2.90 m). A 1:4 balun on a thoroidal core was applied as a feeding transformer.
The author also tested a different antenna feeding system designed to improve the matching characteristics and expand the antenna's operation in the 17 m band.
It is worth noticing that it is a non-resonant antenna, and its WFS ratio is of the order of 2 or more, which requires the use of an antenna tuner.
The antenna was mounted for the first time at about 2 m above the ground level. I could easily make contacts with European amateur radio stations, and that encouraged me to reduce my transmitter's power. I tried QRP (5-10 W) and easily worked Asiatic Russia, Canada, or USA. When I increased the power to about 50 W, I reached Ecuador and Venezuela (in CW and SSB). At that power level, I had my first SSB QSO with Hawaii in the 20 m band. During the 2010 ham radio meeting in Jastrowie, Poland, Andrzej SP9ADU conducted successful contacts in the 30 and 20 m bands. Comparative tests with my Delta and a 7PL Multibander demonstrated a small difference in my signal strength across Europe.
Other tests with my Delta and a Rybakov antenna (placed at 8.5 m) proved that a lower noise level and stronger signal strengths of the received stations characterized my Delta. For that reason, I stopped to use my vertical antenna.