The FXTenna (TM) Portable Antenna 
by Dave Land KD5FX
www.kd5fx.com

UPDATED 9/19/11

Features:
20 through 6 meters, QRP to 100+ watts, lightweight, easy to carry

I've been looking for an inexpensive portable HF antenna for my new (to me) ICOM IC-706 radio. I looked at several models but most were rather expensive. I found several designs on the web to build your own but most involved coils and covered the lower bands. I was more interested in 20 meters and above since the HF bands are coming back now that the solar cycle has finally rolled back around (this is my 3rd solar cycle!). 

I did find one design that used the 10 ft MFJ-1977 telescoping antenna so when I went to look that up on the MFJ website I noticed they also have a 16.75 ft model for just a few bucks more! Hmmm, what bands could I cover with a 16.75 ft antenna? So, I did the math and discovered that it would cover 20 through 6 meters! 

For the base I use a 3/8-24 mobile antenna mount. I got that mount at a local truck stop, in their CB radio supply area. Lots of various 3/8-24 mounts there, I chose one that will work in the yard.  I mounted this mirror mount to a 18" long 3/8" diameter aluminum rod which I got at the hardware store. (Actually, it was 3 ft long, I cut it in half and sharpened then end to a point. I always use the water method to put it in the ground, no need to push hard with that method!)

To use this antenna you MUST set it to the correct length for the band you wish to operate on. Then, when you change bands you just re-adjust the length! The formula MFJ suggests is to use 234/F (Mhz) = Length (feet), or 2808/F (Mhz) = Length (inches) or use the table below.

This antenna covers the whole band with a low SWR and best of all it is very efficient since there are no coils involved! The optimum length for your ground radials are shown in the table below, it will work OK with something shorter, I've tried using both eight 114" radials and eight 204" (17 ft) radials. Can't seem to tell much difference in it's operation with either length. You don't need to retune the radials when you change bands as long as they are the 20 meter length, they will work on all the bands above that as long as they are on the ground. If you use elevated radials you MUST use tuned radials, at least 4 for each band. That's why it's easier to just put them on the ground and there's not need to mess with them again until you pack-up to leave.  I made my radials using WHITE wire, this shows up much better on ground and makes it easier to walk around the antenna without tripping on the radial wires! They are made from 'zip cord' which is sold at the hardware store for small extension cords. Get the smallest gauge they have. I used 18 ga on mine. I put a spade lung on one end and made a small loop in the other end so I can hold them tight with a nail.

Here's an equipment list:

EXTRA-LONG TELESCOPIC ANTENNA,  MFJ-1979 16.75 FT TELESCOPING ANTENNA
3/8-24 mirror mount for CB antennas from the local truck stop (about $10)
3 ft 1/4" or 3/8"  rod from Lowe's for a ground stake (about $5) Cut it down to 18", 3 ft is just too long!
4 to 8 ground radial wires, 'zip cord' works great, smaller gauge wire is OK too, at least 17 ft long for 20 meters and above.
1 large nail to use for the end of each radial wire, I use 16d size, they are about 4" long

Here's a table of the dimensions you need for each band.

Band

Freq

Length (ft)

Length (in)

Amount to shorten 16 ft antenna (in)

No. of sections to collapse 

Amount to shorten 16 ft antenna (in)

Radial length req'd

6 Mtrs

50.1

4.67

56.05

144.95

7 1/2

144.95

4.79

10 Mtrs

28.3

8.27

99.22

101.78

5 1/4

101.78

8.48

12 Mtrs

24.93

9.39

112.64

88.36

4 1/3+

88.36

9.63

15 Mtrs

21.2

10.99

131.83

69.17

3 1/2

69.17

11.32

17 Mtrs

18.11

12.92

155.05

45.95

2 1/3

45.95

13.25

20 Mtrs

14.225

16.45

197.40

3.60

-

3.60

16.87

I add the 'No. of sections to collapse' column so I don't have to measure the antenna every time, I just push all the sections together so it's minimal length and then I DON'T pull out that number of sections. For instance, on 10 meters it's 5 1/4 sections. So I count the first 5 sections from the top and leave them collapsed then I pull out the sixth and slide it down about 1/4 of the way, then extend all the others below that. Makes it easier to change bands that way.

Pictures here:

These are snapshots, click to enlarge and then use your 'back' button to return to this page

This is the FX Tenna set for 10 meters with the white 20 meter radials on the grass

Here it is on 20 meters with the same radials

Here is a detail of the CB Mirror mount on the 3/8" stake. I also have a quick release on the telescoping pole from MFJ to make it easy to connect.

Here is the other side of the mount, you can see how I connect the 8 radial wires, I use a wing nut for quick connection.