AO-40 Portable Antenna

Created 30 April 2001

Updated 5 May 2001

The idea of this is to allow me to take the existing split boom Arrow Antenna and allow the option to use it on 23cm and 13cm as well as keeping the ability to use the antenna on 2m and 70cm.

Four new combinations, clockwise from top left:Mode US high gain; Mode LS high gain; Mode US low gain; and Mode LS low gain

23cm was added onto the boom by disassembling parts of and an existing 23cm yagi and drilling out matching holes into the split Arrow boom. The 23cm is in the same plane as the original 70cms, so optionally the 23 cm uplink can be replaced with 70cm. The original 23cm antenna was a 19 element Sandpiper unit. The elements were marked with an indelible pen in order to distinguish them for reassembly.

The 13cm antenna is a www.maplin.co.uk 15 element yagi (part number DS30H) normally used for TV survaillance. It is attached to the Arrow boom separately. As the elements on the 13cm yagi are precision made at this frequency I chose not to disassemble the antenna.

The smaller 13cm antenna (only 8" from reflector to last director) is available from www.g1mfg.com 

13 Elements on 23 cm, 15 elements in 13 cm in the front room antenna range. The picture is deceiving: the antenna is actually much smaller than the sofa.

 

The elements are deliberately placed in different planes as I found that significant desense occurs on 13cm with 10W transmitted on 23cm if the antennas are in the same plane. Spacers and long M3 bolts are used at the handle end of the 13cm antenna to affix to the Arrow boom.

'Nice day at the office dear?'

 

Testing the configuration - including desense

I discovered that the Diamond SX-1000 also works quite well as an SWR meter at 13cms too, even though it's not rated for that frequency. I'd take the power readings with a pinch of salt - probably 30% optimistic. I got 1.3:1 SWR on both the 23cms antenna and the 13cms antenna.

FT-736 10W 23cms TX. DB6NT 1W 13cms Transverter. FT-817 IF. Diamond SX-1000 power meter. IC-T81 (not shown) derestricted to work at 480MHz (5th harmonic=2400MHz).

Whilst transmitting 10W CW on 23cms, I wondered around with the IC-T81 transmitting FM on 480MHz (into a dummy load of course), which turns out to be quite a wide band weak signal source by the time it hits 2400MHz. This allowed empirical tests of desense of 13cm from the 23cm transmitter. If the antennas were in the same plane the desense was very marked. Putting the antennas in perpendicular planes immediately resolved this problem.

Mail Howard, G6LVB

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