Monday, July 1, 2013

A linearly-loaded, trap dipole for 80, 40, and 30m

Since I have had my hex beam up, I have not been operating much below 20 meters.  The band conditions have been terrible anyway.  Still, it has been a goal of mine since then to get up a antenna for the low bands that does not require a tuner.  My doublet was working fine, but I no longer needed the multi-band capacity.

This lead me to a number of choices.  The most obvious was a basic dipole for 80m, or a fan dipole perhaps.  Neither of those options really sounded all that great though.  The way I have my antenna farm set up, I don't have room for the full 125 feet I'd need for an 80m dipole, so I'd need something with some loading to fit in my available space. Secondly, I wanted 40m, primarily so I could use my QRP rigs without a tuner, like the MTR.  As a bonus, I figured I'd might as well include something for 30m as well.

Wishing to try something new, I decided on a trap dipole for those three bands. AK0M, Steve, works in my building and he has had lots of success building traps for his EFHW antennas he uses at home and for portable ops and he suggested I give it a shot.  I had not built any trapped antennas before, usually opting for one-size-fits-all balanced line fed doublets and relying on a tuner to do all the work.  He pointed me to an app from VE6YP which can be used to quickly design coaxial traps.  His site is here.  Simply input the type of coax you're using - there are several built in, if yours isn't there, you can look up the specs you need in data sheets - and it will churn out the turns you'll need for each trap. Based on some research, it is suggested to build your traps for resonant frequencies slightly below your operating frequency.  The one article I read had a trap for 40m tuned to 6.5 MHz.  I didn't look much into it so I made my traps for 6.5 MHz on 40 and 9.6 MHz on 30.  I built my traps out of 1 1/4" 1120 PVC pipe.  1120 has a very thin wall compared to schedule 40 PVC and is perfect for making traps since it is lightweight.  I used RG174 coax that AK0M gave me for the coax elements.  The schematic for the traps is shown below:

The tuning of the traps is somewhat critical, so you do need test equipment for this project.  An older MFJ antenna analyzer is suitable though, and are usually easy to borrow if you don't already own one yourself, which is what I did.  You can see a video of how to use the MFJ analyzer to determine the resonant frequency of the traps as you build them.

As for the loading, I chose a linear loading method covered in a QST article from July 2002 issue by K4VX.  
  Its available without a subscription.  In the article, K4VX builds a 40m dipole using 450 ohm ladder line to linearly load the antenna, resulting in a 30% reduction in total length.  I scaled the lengths for 30m and built mine up, allowing for a fair amount of length for tuning purposes.  I didn't keep track of my lengths but you can calculate them easy enough.

I proceeded to hoist the antenna up to the top of my mast to tune the 30m section.  Remember to tune WITH the traps in place, since they do add a bit of loading.  A few trips up and down and the 30m element was tuned in just fine.  After reading K4VX's article, I basically left the loading section (the 450 line) alone and limited my trimming to the 30m wire, this is important and you'll see why soon.

Being unsure where to start with the 40m wire, I measured from the center, 33' as if I were making a full size 40m dipole.  Of course it resonated far too low, at nearly 4 MHz!  I started lopping off several feet at a time until I zeroed in on 7 MHz.  As I got close, I checked the 30m resonance and found that it moved down in frequency a bit and was no longer in band.  This is where I started trimming the loading section.  I took 3 inches off at a time and pulled the 30m part into range again while trimming the 40m section too.  This took a really long time. I was being extremely cautious to avoid over-trimming since it was looking like the usable bandwidth on 40m was going to be close to only 70 kHz.  A bit too short, and my goal of a no-tune antenna for 40m CW would be lost.  Finally both 30m and 40m were in tune and I added the wire I had left over to the ends for 80m.  Again, both 30 and 40m sections were slightly affected by the additional wire so I trimmed another 3" off the loading section, and a bit more off the 40m section.  A couple more trips up and down the mast awarded me with 3 bands in perfect harmony, providing <2:1 SWRs in the spots where I wanted them.  HOORAY!
A piece of one side of the antenna, the center is at the mast under the hex beam.  You can see coils for 40 and 30m here.
It was about this time that I realized how short this antenna was.  The doublet it was replacing was about 88' long when I took it down (it was 100' when I put it up, before I trimmed for that "magic" length).  In comparison, this thing was dinky.  I suspect I could have done without the loading all together, but we live and we learn!  I just hope the efficiency does not suffer too badly.  Time will tell.  The good thing about antennas is that nothing is permanent, we can always put something else in its place!

I have not had sufficient time to put it through its paces, and with the band conditions as poor as they have been lately, now would be the worst time for a real assessment of its performance.  Keep an eye on my twitter (@w0ea) for up to date reports from the shack.

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