martedì 7 gennaio 2014

Building a Short "Longwire" antenna

Ok, I'm not kidding: I'm really trying to shorten a longwire antenna.

The problem: a newcomer want his first antenna, want 40-10 meters at least, have no space, have a fishing rod only 7 meters long.

For first antenna I usually suggest at least 9 or 11 meters of wire and a 9:1 un-un. This not the most effiective antenna to build, but is a simple to build and low cost multiband. But 7 meters of fishing rod....

If you look at the table in my older post on 9.1 iun-un you find two suggested lenghts shorter than 9 meters: 6.5 and 7.5 m, but I was not able to maych it on 7 MHz using either LDG Z100 and Elecraft T-1, the minimal useful lenght for 7 MHz is 9 meters!

The only solution seems to shorten the antenna. But the "longwire + 9:1" is yet a compromise antenna, loading coils may lower further the effectiveness of the system, resulting a long dummy load!

As a last chance, I'll try to shoirt the antrenna wire using the reduced velocity factor of coaxial cables.

An RG58 cable has a velocity factor of circa 0,67: This means that for reach an ELECTRICAL lenght of  9 meters I need a PHISICAL lenght of 9x0.67 = 6.03 meters of RG58.

This is a known trick, for an example of using velocity factor for reducing wire lenght look at this project in Ham Universe.  You may read also a more accurate description of the effect of velocity size based antennas on G3TXQ web site.

The coaxial cable MUST be connected in this way: on one side of cable you connect only ONE conductor (central or braid); on the other side, connect braid and central togetther.

I'm able to obtain a match also in 40 meters with LDG Z-100 tuner.

Last news: the friend which is using this antenna reports suffcient performance, many european QSO also on 40m.

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