eHam Forums => Elmers => Topic started by: AB1XG on August 10, 2005, 03:49:37 PM

Title: Dipole for 17 Meters
Post by: AB1XG on August 10, 2005, 03:49:37 PM
My current dipole will tune 10 meters and 20-80 meters.

This is good, but I miss 17 meters! It's one of my favorite bands!

So I had enough junk around to build one for free. Though I prefer ladder line, this one will be coax fed. I can live with this for FREE.

I calculate each leg should be 12.9 feet long, or 12 feet and 11 inches or so.

Only problem is the SWR is 3:1 at 18.110 and between 2.5:1 and 3:1 at 18.168.

Gee, is this thing too short or too long??

David, N1ZHE

Title: Dipole for 17 Meters
Post by: WB2WIK on August 10, 2005, 04:16:38 PM
Impossible to answer that, given the data.

You need to sweep the whole band from 18.068 to .168, plot SWR vs. frequency and see if there's any trend.

If there's no trend at all, either it's way too long or way too short -- or, possibly the antenna is unbalanced so even if it's the correct length it still won't match well.  That's a common problem with dipoles or inverted vees that are "too low," and coupling to the earth or objects beneath them.

One great reason for owning an antenna analyzer, as opposed to just an SWR bridge, is it allows you to "sweep" the whole HF spectrum looking for resonances.  It may be your dipole "dips" just fine at 17.935 or someplace, but you'd need an analyzer to find that -- a transmitter and SWR bridge won't do it.

But at least look for a trend.  Making measurements at only two frequencies isn't enough.


Title: Dipole for 17 Meters
Post by: X-WB1AUW on August 10, 2005, 04:43:18 PM
Will it be easier for you to add wire to each end, or to shorten them?

Alligaor clips to lengthen; fold the ends back onto the wire to shorten.


Title: Dipole for 17 Meters
Post by: KZ1X on August 10, 2005, 07:16:11 PM
EZNEC shows a bare 12 gauge wire dipole up 35 feet needs to be 13 feet 2 inches on a side, and this yields 72 ohms nominally nonreactive at 18.07 MHz

Title: Dipole for 17 Meters
Post by: N6AJR on August 10, 2005, 07:23:22 PM
My guess is it is too short, it seems to be slight lower-ish  at the high end so probably a bit short. try aligator clips with 1 foot each end and then trim and check..

Title: Dipole for 17 Meters
Post by: K7PEH on August 10, 2005, 08:52:45 PM
I just ran Eznec on 12.9 feet for each leg of the dipole up 35 feet and center fed with 50-ohm coax.  This is with a 101 segment count.

The SWR results are as follows:

18.11 MHz, SWR is 1.5

18.31 MHz, SWR is 1.21

18.68 MHz, SWR is 1.76

Minimum SWR is indeed at 18.31 MHz.

Other stats resulting from the computation are:

Impedance Z is 59.37 - J 4.549 ohms  At 18.31 MHz

So, it looks like the antenna is OK.  I would use it.  You may want to take the TLW program (available on the ARRL Antenna Book CD) and feed it the length of your transmission line and the type (RG8X for example) and specify the load impedance of the antenna that I included above.  This will give you the impedance that your transmitter or tuner sees.

I used an average Real/Mininec ground.

Title: Dipole for 17 Meters
Post by: WA6BFH on August 11, 2005, 09:47:07 AM

I love these comments about EZNEC! I have never to this day, over the 20 some years since another Ham enthusiastically told me of this sort of antenna MODELING program, seen an actual antenna that turned out to match what the program predicted! You simply don’t know the real world conditions as to how the antenna is fabricated and installed.

The first “computer” that I used to calculate the APPROXIMATE  dimensions of my first dipole (about 40 years ago) was a Slide Rule. Later I used an electronic calculator. In all instances a second valuable tool was employed. This was a pair of Diagonal Cutters, to trim the antenna to resonance -- in the real world!

Title: Dipole for 17 Meters
Post by: N6AJR on August 11, 2005, 12:35:43 PM
I am a great rule of thumber.. I use 33 feet per side  on a 40 dipole, now for 20 its 1/2 that or 16 1/2 feet per side,  for 10m its a little over 8 feet 3 inches each side,  so 15 m would be about 12' 4"  and that makes 12m about 10'2 " and 17 m would be  14 feet 6 inches. of course 80 is 66 feet a side, and 160 is 121 feet a side.

these are all starting places, and tend to be a bit long, but its easier to wrap the wire to make it shorter than to make it longer.  or put them all on 1 antenna and call it a fan dipole, (phan Dypol?),

thses will be shorter if less than 1/2 wave above ground and if 1 under the other there will also be interaction, but you can run them  1 notrh and south the next east and west and the next caddy corner one way, and the next caddy corner the other way etc , to cut down on interaction between them
 I agree that  a modeling program is a great toy to look at, but the good old rule of thumbs works for me