RAN Technology

My SWR Dilemma


Vintage Ham Radio 0 Comments 04/13/2021 

Learning about trap dipoles and SWR

Posted By: Robert Nickels (ranickels)

 

When I got my Novice license in the fall of 1965, my dad had also decided that as long as he was taking me to the classes at Blue Valley Amateur Radio Club in Seward, Nebraska, he might as well try for his license too.    I was fortunate that not only were my parents supportive of my ham radio interest, but my dad could build anything.   So the first thing was a 40 ft. mast to support an antenna, which turned out to be a HyGain 2BDQ trap dipole.

 

 

Like most new hams, we didn't know anything about antennas or how easy they were to make, and the lure of two-band antenna from a company like HyGain in nearby Lincoln NE was strong.   I suspect we showed it to the guys at the club who said "Yeah, that outta work" and that was it.   The antenna worked like a champ for SWLing and following the instructions in my HT-40 manual I loaded it up and made contacts on both 80 and 40 meters.

For Christmas I received a Heathkit HM-11 Reflected Power and SWR Meter that by then had transitioned to the Heathkit Green color scheme:

 

 

Not that there was much to putting it together but my success rate with my homebrew projects was maybe 50-50 so I was pleased that the meter wiggled and it seemed to work.   But then I was quickly disappointed after finding that it said the  SWR on my trap dipole was 4:1 or so - in fact it was in the RED!  Above 3:1 the scale isn't even calibrated, and if more than 33% of my power was actually being reflected back to the transmitter, I figured it couldn't possibly be working as well as it was.   My conclusion was that I'd screwed something up and my new station accessory was in the unfortunate 50%.

I continued using the trap dipole through my 9 month Novice career but sometime within a year or so (I can't remember if I'd upgraded to General or not) - I was talking on either AM or SSB to a ham travelling through town who offered to stop over and take a look at it.    He connect his SWR bridge to my antenna and guess what it said?   4 to 1!    My problem was with the antenna not the meter after all.

Now HyGain must have had a good reason for using the extremely stiff and springy wire they did, but after a couple of attempts to lengthen or shorten it (confounded by the fact that it was a trap antenna so changing the length of one section also affected the other band) - my antenna advisor asked "Do you have any wire?".    My dad always had 12 or 14 ga. copper wire around so we made the executive decision to cut the trap dipole loose and measured out plain old copper wire in lengths correct for an 80 and 40 meter fan dipole.   Because the ends came down in an inverted-vee to our fence it wasn't long before we'd tweaked both lengths and the SWR bridges agreed it was less than 1.5 to 1 on the part of the band I was most interested in.    And I'm sure I made even more contacts!

When you're starting out at anything it's pretty inevitable that you'll learn the same lessons other have learned but never think to tell you about.   Such as the fact that even an imperfect antenna will still radiate well enough to make lots of contacts.   And that there is much more to the subject of transmission lines and standing wave ratio than you pick up from a magazine article or two.     Now I'm sure the 2BDQ is a fine antenna, otherwise it wouldn't still be in production, but for some reason it resisted our efforts to optimize it and the notion of doing a couple of simple calcuations using the formula I'd memorized for my test and actually making a wire dipole that did just what it was supposed to was very satisfying.

One thing I'll say for the HyGain - the center insulator was the best ever!

 

 

And to close this story out - although I no longer doubted it's accuracy, at some point a few years later I determined that a green SWR bridge just did not have a place in my shack, so I gave it a make-over in two-tone grey and with modern knobs: