Portable Resonant Antenna for QRP

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I wanted to build a new and smaller portable resonant antenna to use for QRP. I am not using such a light wire as I initially thought about. The reason for that is that sometime in my area the winds are pretty strong, so a thin antenna wire would not last long.

The 40m and the 20m bands are the ones I use the most, so this antenna is perfect for me. But if you want, you can build it for the bands you desire. In the future I may build another one very similar to this one. It will be an 8010 EFHW that is a lot longer and has a compensation coil. This one will be very similar to what I currently use at home, but a lighter version.

THE 49:1 IMPEDANCE TRANSFORMER

In the previous article I described in detail the way I usually make the 49:1 impedance transformer for my resonant EFHW antennas. This one is no different, except that I decided to build a custom enclosure for it from PVC pipe. I wanted something watertight, experimenting with building enclosures from PVC pipe.

For the impedance transformer I used an FT 114 – 43 toroid together with the 1mm enameled copper wire. The value of the capacitor is 100pF at 1.5KW. I picked the diameter of the PVC pipe in a way to also fit the toroid inside, leaving some space around it.

Once I soldered all the parts in their place, I glued PVC covers on both sides of the PVC pipe. I used 2 turns for the twisted primary and continued with 12 turns for the secondary. Note that the PVC pipe walls are double. I basically cut another piece and glued that inside to strengthen the entire PVC pipe enclosure. Check the 49:1 Impedance Transformer article for a few mode details about it.

Here you also have an image of the finished 49:1 impedance transformer. All I did extra was to spray it with a few coats of clear varnish for better protection against rain. On one end of the transformer there is a BNC connector.

One side has a simple hook so I can attach the rope that will hold one end of the antenna. The other side has a screw – in type of connector for the antenna wire. Except the toroid, the entire impedance transformer was made using recovered parts. I am thinking on rebuilding this in a bigger enclosure that will allow me to use bigger ferite cores.

With the FT 114-43 there are losses as well so the antenna is not as efficient as it is with other cores. Also I would not use this impedance transformer with more than 15W. But for portable QRP should good enough. I built with what I had on hand. Also the turns number can be changed to make the antenna more efficient ( not the way I built it here ).

THE PORTABLE RESONANT ANTENNA WIRE

As I was saying, I decided to build the antenna for the 40m band. Knowing that it will also resonate well on the 20m band, I was more than happy. The only thing I was scared about, was that I didn’t know if the SWR will be good on both bands. It turns out that the SWR is lower than 1 to 1.1 on both bands, so that it’s perfect.

The low SWR is only on the center of each band, as it will go up to about 1 to 1.2 – 1.3 a the edge of each band. But I’m happy with that. I have a feeling that if I will raise the antenna even higher, the SWR will also be lower on the edges of each band. We will see about that in the future.

I started with a wire length of 20.5m, knowing that I will trim it until I get a good SWR on the 40m band. More exactly, I tuned the antenna for the SWR on 7.1MHz. By doing that it seems that I also have a good SWR on the 20m band. With the help of the 100pF capacitor, the SWR on the 20m band seems to be more even on the entire band. That wasn’t the case with ought the 100pF capacitor. I also tested using a 150pF capacitor, but the SWR was not so good as expected.

First I thought about building it very light with a thin wire that has teflon insulation. In my area sometimes I have strong winds. So I decided to go with something robust. I ended up using a 1.8mm wire in diameter with the insulation. At both ends of the antenna, I prepared 10m of rope to hook it up in a tree. It turns out that in the side going up in the tree, 10m of rope was not enough. So I ended up using 25m instead.

Since the 49:1 impedance transformer can be used for other bands as well, I will try to experiment with the wire part of the EFHW antenna. Since I have the (tr)uSDX Transceiver, I will try to build a multiband EFHW antenna using traps ( or maybe quick connectors ). This way I want to build it for all the five bands I have available in the TRX. But for now I’m really happy with the one I have. It’s my first design for QRP portable work and I’m glad it turned out so well. I was not expecting the SWR to be so good as it is. The nanoVNA saved me a lot of headaches trying to tune it ( thanks to my sister and brother in law for the nice present ).

ATU vs 49:1 IMPEDANCE TRANSFORMER

Play Video about ATU vs 49:1 Impedance Transformer

I was curious if for portable work I should use the 49:1 impedance transformer, or an ATU ? Here you have a quick comparison using the same 20m of wire antenna, as a half wave 40m band antenna. Turns out in my opinion that an ATU may be more reliable helping me get more bands out of the same antenna. On the other side, the 49:1 impedance transformer is smaller in size and it does not need constant adjustments when changing bands.

SWR READINGS

Now all I need is some more free time and good weather so I can go portable. I want to try some SOTA activations using this EFHW antenna together with the (tr)uSDX Transceiver on SSB. So I hope to get some of you in my log . About 11km from me I have the “Pilisca” peak at about 1223m altitude ( YO/EC-271 ). This will be a special SOTA activation for me since during my childhood we used to go and cap there overnight. So I want to collect some more great memories in that place to the ones I already have. Above in the video you also have the SWR readings of the antenna using the 49:1 impedance transformer, compared with the Antenna Tuner that I also build some while ago ( still experimenting with the design ).

73 DE YO6DXE

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