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vertical antenna wire length? (2 replies)
Reasonant on lower frequency so cut length of 16 feet 6 inches by 14". Swr now at 1,1 across ful 20m band. formila os 234/frew ammended to 218/freq and perfection. Making qrp contacts 1500 miles away. Constant of 218 arrived at by multiplying frequency of best swr by antenna length then using new constant (218 in my case) and dividing by frequency required to give new resonant length of vertical wire. Simply cut or add wire to existing antenna to give resonant length. Well pleased with myself. (my calc. was best reasonance of 16feet and 6 inch wire at 13.198MHz giving constant of 218 which converts to 15 feet 4 inches for 14.175MHz)
73 (72 to qrp users)
SWR will be affected by other objects nearby, size/thickness of wire, velocity factor, local bird life, 'r' in the month etc! I find best thing is to start with a slightly longer length than calculated then trim down to match.
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Using the many on line calculators I decided to build a vertical 20m antenna with wire up a telescopic pole. Length worked out at as 16 feet 6 and 1/8 inches.
However a Nano VNA gives swr as 2.3 so lenthenned by a further 6 inches bringing swr down to 1.3 and a contact with LZ1QN/P in Bulgaria some 1608 miles with 10w. Would like to improve on this but sceptical of calculation formulae (234/freq. for 1/4 wave vertical). Is there a better formula to get a better swr other than the ones I am using which seem to get it wrong. Lengthen or shorten is my other query. Which way do I go if swr at lowest on 14.300 and highest on 14.100.
Sorry if this is basic stuff but recovering from 2 recent strokes which have stopped progression into Intermediate or Full due to memory problems. 10w will do for now as I can still get on-air although an efficient antenna is a must for this with as low an swr as possible, hence my need to know how to either cut or add.
73 to all out there
Doug MM7DSA