Resonant Feed Line Antenna for 40m

After my success with the Flower Pot antenna on 2m I was inspired to try the same antenna design on the 40m (7MHz) Ham band. At HF frequencies, my ARRL handbook calls this design a Resonant FeeD line (RFD) antenna.

RFD 40m antenna

I used the dipole formula of antenna length equals 143/f meters where f is the frequency in MHz. This came out to 20.14m at 7.1MHz, so I made the coax 10m, and the wire 10.1m. The 1995 ARRL handbook design specified 15 feet (5m) of coax coiled into a RF choke of 6 inches (0.15m) diameter for the 40m band.

Due to the weight of the coax, this antenna in heavy. Especially compared to inverted V dipoles where the centre mast supports the coax feed line. For inverted V antennas I can get away with light duty speaker wire. So for this antenna I made a strain relief to support the choke (4 notches cut in a piece of wood), as I didn’t think electrical tape would hold the coil shape under the strain.

RFD choke and strain relief

I am a bit nervous about the stretching strain put on the coax. Also compared to an inverted V dipole, with this antenna I need a mast at either end of the antenna to get a good height. However the antenna is much more horizontal, which might help the radiation pattern.

Here is the feed line end, mounted on a mast on top of the house. I now have a 2.4 GHz Mesh Potato, 2m Flower pot, and the RFD 40m HF antenna.

2400, 144, and 7 MHz antennas

I have the antenna suspended from a pole about 7m above ground at one end, and from another mast 5m high at the other end. The SWR on 40m between 7.0 and 7.2 MHz looks OK, and was also OK on 20m which was a nice bonus. The SWR is even low on 2m! On the first evening I could hear stations from Europe and Australia, so I guess it’s working OK – apart from my terrible background noise problems….

RFI/EMI Noise

My background noise level on 40m is a very high S8. At once stage I traced this to the co-located Mesh Potato, which I think it is radiating at HF via the long Ethernet cable I use to supply power. Switching off the Mesh Potato reduced the background noise level to S5, and removed “birdies” on other HF and VHF bands. A ferrite clamp on the Mesh Potato Ethernet cable just after it leaves the MP enclosure might help reduce the RFI.

I also found our Electric Car to be a source of RFI on 40m and 20m when it is charging. This consists of several high current switch mode chargers (around 1500W total), and long wires through car connecting all the batteries together. EMI from EVs will need some serious attention in the future as they become more common. It’s probably much worse when they are driving, e.g. 50-100A currents at 120V being switched off and on quickly using PWM controllers.

However even with the EV and Mesh Potato off a few days later some other source of noise was back – once again at S8 level on 40m. Switching off all of my office equipment didn’t help. Not sure how I will track this down, I might need some form of noise cancellation.

3 comments to Resonant Feed Line Antenna for 40m

  • David,

    If your 40-meter 1/2 wave dipole has low SWR on 20 meters, I think something is terribly wrong. Maybe I don’t understand that antenna, but it seems to me that a dipole made of a half-wave on each side is going to be very high impedance at the feed point. You might be getting some lucky transformer action out of the length of your feeder.

    OTOH, the 40-meter dipole should work very well on 15M. But on 20 it should be terrible.

    73 de n1kdo

  • John Bache

    I too have made up a 40M RFD, & with the aid of an old MFJ ATu I can get away on most bands (including 20M. Three cheers for the ARRL handbook.

  • John Moore, KK5NU

    Happened to stumble across your Codec2 Presentation at the Ballarat Linux Conference last week –

    Grateful thanks for *all* your efforts – Let’s hope we get over the WMD hurdle on this one.

    72/73 — John, KK5NU

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