Antennas for Portable
Simple dipole antennas;
When operating low power / QRP, one should maximize their antenna. In one of Bill Orr’s (W6SAI) antenna books, he said, “a resonant antenna radiates the most RF” and I’ve lived under that premise.
When I first set up portable, I used a simple coaxial fed dipole.
Sometimes a full size dipole just won't fit so I decided to make a few shortened dipoles.
Shown below is a 12 foot long 20 meter dipole. This antenna is resonant in the CW part of the 20 meter band. It can handle up to 150 watts. The coax, with a #31 clip on ferrite bead, is 25 feet of RG 174. The ferrite bead has three turns of coax wrapped around it. It is a quick and simple current balun.
My two favorite bands were, and still are, 20 and 40 meters. So I next changed to a dual band 20 / 40 dipole. Overall length for the dual band dipole is only 44 feet long and is perfect for portable operating. I use half of the above antenna to make a dual band vertical wire antenna used on my van. This is shown on a different page.
Off center fed antennas:
After having great success with simple dipoles, single band and dual band, it was time to try something new. After doing a lot of research about off center fed antennas, I decided to try an off center fed 40 meter antenna. What I really liked about the ocf40 is the amount of bands you can cover with one antenna. It covers 2, 6, 10, 20 and 40 meters and it is in fact resonant on all 5 bands.
The picture on the right below shows the actual SWR plot of my 40 meter off center fed antenna hung at the 35 foot in my back yard.
The picture on the left below is an off center fed 40 meter antenna. It is compact enough that it will actually fit in my pants pocket. This antenna is strictly used for portable operating.
Half wave end fed antennas:
I then decided to look into half wave end fed antennas. Since they are actual resonant antennas on numerous harmonically related bands, they looked promising. I built a 64 to 1 UnUn and cut the antenna wire to a length of 65 feet 5 inches. I was impressed with the results. The antenna was actually resonant on 10, 15, 20 and 40 meters. Like all of the other antennas I have used before, I did not need a matching network (ATU) between the antenna and the radio. This made it great for fast band changing.
The picture above left shows my first, and still used, half wave end fed 40 meter antenna. It is rated at 50 watts. The coax I use to fed this antenna is 25 feet of RG-174. On the end of the coax, closest to the radio, is a clip on #31 ferrite core with three wraps of coax through the core which is used to choke off the RF that will show up on the outside of the shield of the coax. When more coax is needed to reach the radio, I add another 12 feet of RG-174 also shown in the picture. The picture to the right of the HWEF40 is a Rig Expert AA-600 showing the SWR plot of the antenna.
When operating portable, all of my antennas are erected in a horizontal plane as high as I can get them which is not high at all. Most of the time they are between 15 and 26 feet. I've worked a lot of DX with only 5 watts into these antennas.
I've done A/B tests comparing my ocf40 against my hwef40, using reverse beacon and tests have shown that they are equal in performance.
Shown to the right is my micro half wave end fed 40 meter antenna. This little guy will fit into my shirt pocket. It will handle up to 25 watts. The wire is 22 ga stranded teflon coated. It uses a 64 to 1 UnUn and is built on an FT 114-43 core.
Mouser.com is a good place to get ferrite cores at a very reasonable price. Here's the info for an FT 240-43 core. The picture on the mouser web page is deceiving. Here's the actual data sheet for the above Fair-Rite core. The part number on the top of the page is the actual Fair-Rite and mouser part number.
Here's a good place to get teflon wire.
End fed antennas are notorious for RFI issues. Why? Because the outside of the shield of the coax becomes the second part of the antenna. You must decouple this before the coax enters the dwelling. Attaching a ground rod to the UnUn does not resolve this problem. A ground rod gives lightning a path to ground, it is NOT a counterpoise for RF.
I addressed this in the attached PDF article on end fed antennas.
For more information on end fed antennas and how to make them, read the following article: End Fed Antennas.
BNC / Low power antenna switch:
I wanted to do some antenna comparisons between antennas and needed an antenna switch. Since I like keeping everything small and compact, also I only use RF connectors that are BNC, the hunt was on for a switch. I could not find one that used BNC's so I had to make my own.
Below left is an inside view of the switch. I shunted a 12 pf capacitor across the input of the switch to cancel out some of the inductive reactance created by the actual throw over switch and the internal wires. This helps to keep the SWR insertion very low on the high end of the frequency range. The isolation between ports is around 23 db depending on the band of operation. It's great for switching between two antennas but not suggested to be use with one antenna and two radios if you plan on transmitting.
This antenna switch can easily handle 200 watts and can be used in the primary station for low power HF radios. I would not suggest using the above switch on frequencies above the 6 meter band.
Parts for the antenna switch were purchased from Marlin P. Jones.
The aluminum box is part number: 16281 BX
The BNC hole mount is part number: 20507 RC
The SPDT mini toggle switch's part number is: 31881 SW
The 12 pf capacitor is optional and can be purchased from mouser.com
I use a Brother P-Touch to make labels.