a short 40m dipole

March 30th, 2009 by g4lds


The GM4JMU Shortened 7Mhz Dipole

When looking around for a 40m diople, found this and built it I can say it works even if short!  I got a good match with my MFJ analyser and on QRP, worked around Europe ok.  I also heard VK/ZL!  And only at 30 feet as an inv Vee so if stuck for space try this.
To construct each side of the antenna proceed as follows. Cut a 10.25 metre length of 24/0.076 insulated wire, and a 160 mm length of 40 mm o.d plastic tubing ( white plumbers tubing). Measure a 2.75 portion of the wire and attach the wire to the plastic former. Wind 40 turns of the wire onto the plastic former, and firmly secure the end of the winding. Make the other half of the antenna in the same way. Attach the ends of the 2.57 metre sections to a suitable centre insulator, which should also mount the choke balun, connect the 50 ohm coax, then carefully waterproof the whole assembly.
The choke balun uses RG174AU coax and a 40mm Ferrite Toriod.
Once the antenna is erected adjust it to resonance on 7.030Mhz (or wherever your fav 40m freg is!) by folding back the ends, and adjust the length to provide minimum SWR.

This could be made multiband by replacing the feed point with a 4:1 balun and feed with 50 Ω co-ax or directly with 300 Ω (or 450Ω) ribbon feeder to a balanced AMU or 4:1 then AMU. (update 8-09-10)
Article from SPRAT Issue 74 Spring 1993


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Elecraft K2 Internal 4:1 balun

March 8th, 2009 by m0rgb

Whilst looking around at the various mods done to K2's I discovered the following balun.

It is a small efficient QRP balun designed some time ago by Charles Greene W1CG. see www.njqrp.org for further details, including full building instructions.

My K2 is setup on internal batteries with it's main goal to be a portable radio. As I use mainly doublets through a 4:1 balun I thought building this inside the K2 would suit my needs perfectly.

I purchased the toriods from www.qrpproject.de you can buy these ready made but I wanted to build it! The toriods purchase were Amidon FT82-43.

Main components used

Main components used

As I had already performed a mod to add a separate charging socket to my K2 I knew I would have to move this to allow me to add the balun.

The balun consists of two toriods wound with 6 turns on way, a cross over and 6 turns the other way. Then wired together to give 4:1

Winding was pretty easy compared to the many toroids in the K2 :-)

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