Monday, May 6, 2019

Commercial Log-Periodic Antenna for 30 to 70 MHz

One of the challenges of listening to Low Band VHF signals is having an antenna that performs well on a wide range of frequencies. One solution is the Log-Periodic antenna.

This 7 element Log-Periodic from M2 Antenna Systems is very much a high product aimed at commercial, scientific and commercial agencies rather than the hobbyist and probably has a price to match. It does however give some insight into that is required to cover the whole spectrum from 30 to 70 MHz.

Some notes and observations...

1) Boom... The boom is 157 inches in length which is about 4 metres.

2) Gain... Even though the Log-Periodic has 7 elements, the gain is very modest at about 4dBd. Note that this is true gain in free space and is honest. They haven't inflated the figures by adding on ground gain to make the antenna look better.

3) Compromise... The modest gain is the price of having a wide band antenna. With a 7 element Yagi, all 7 elements are in use and the gain may be up around 10dbd. With a Log-Periodic, only about 2 to 3 of the 7 elements are active at any particular frequency in the 30-70 MHz range.

4) Phasing harness... Note the phasing harness above the boom which goes back and forth between elements. This is part of the additional complexity of a Log-Periodic as opposed to a Yagi with its parasitic elements.

5) Balun... This antenna has a 4 to 1 Balun at the front and this performs two functions...
a) This is to match a balanced antenna to an unbalanced coax feed
b) It also transforms the 200 Ohm feed impedance of the Log-Periodic down to about 50 Ohms.

More information on the antenna can be found at the following link...

1) 30-70LP7 on M2 Antenna Systems website

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