Easy to Make Doublet for QRP

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

Previous topic - Next topic

bkmoore

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.

73,

Brian

gil


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.

Lamewolf

I made a similar antenna for portable operation but I made my elements 44' long for a total doublet length of 88' and fed it with lightweight 300 ohm TV twinlead and my little homebrew Zmatch tuner for 10 thru 80 meter coverage.  The wire elements on mine was made from insulated #20 speaker wire.

gil

Hello, I have yet to try one but it's on my TODO list...

Gil.