Easy to Make Doublet for QRP

Started by bkmoore, April 11, 2020, 01:48:10 am

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April 11, 2020, 01:48:10 am Last Edit: April 11, 2020, 01:49:45 am by bkmoore
While browsing the NorCal QRP web site, I stumbled upon this very nice design for a portable doublet antenna: Norcal Doublet

Since I operate portable QRP, I am always looking for new antenna ideas, and up to now have never found a center-fed dipole that I was happy with. I ordered 65 feet of 4-strand ribbon cable for $9.99, made end insulators from an old paint mixing stick, and used a few odds and ends from my junk box. I made the elements each 22 feet in length, per the Norcal design, but left the remainder of the cable to be a feed line, so that I could lengthen the elements later on if I want to. I attached one end to a tree, and used two collapsable poles for the center and other end. The antenna was perfectly horizontal at 20 feet. Setup took only a few minutes.

I used a QRP-Guys Multi-Z tuner, which can take balanced line. I was able to tune the doublet on 40, 30 and 20 meters. Using my 5 watt, 40m QCX transceiver, I made an initial QSO with someone in San Diego, distance 246 miles. He reported my signal at 599. I made a second QSO into Utah, distance 310 miles, reporting 559. This was mid afternoon, so the band hadn't completely opened up yet.

I really like this antenna. It is it is very light, easy to setup, the feed line loss is minimal, and so far has given me very good results. I may lengthen the elements to 33 feet each, which is close to the recommended length for a 40m doublet. I will also make a 4:1 balun, so I can connect it directly to my K2's auto tuner.

For me personally, my ideal field setup would be a combination of a vertical, such as the QRP guys trio-band vertical, and this antenna. This antenna would also work very well in a stealth installation.




Old Radio Bloke

You can't go wrong with a doublet. It has been a favourite multiband antenna for many decades for good reason. It's not resonant but (assuming it is designed/built correctly) it is very efficient.

I haven't tried the NorCal idea of using computer ribbon cable. However, I did make a QRP/P doublet with radiators each 5m long. For the feeder, I cut up a large plastic milk container into rectangular strips about 15mm by 30mm. On each strip I made a hole near each corner and fed thin multistrand wire through the holes so that they acted as spacers. I guesstimated the impedance of this homebrew ladder line was a few hundred ohms (exact value is not critical).

The ladder line is about 10m long and is very light and flexible compared to commercial window line.

I used it with a GQRP "Sudden" ATU which has balanced and unbalanced outputs. The antenna worked OK on 80m to 20m. In fact, I recall at least one SSB contact on 80m running just 5W.

I reckon this is a good prepper antenna as it works and is easy to make? All you need is some wire and an empty milk carton.