Discussion in 'Amateur Radio News' started by KB7TBT, Dec 9, 2019.
Ham Radio - The Doublet. An old but good multi-band single dipole antenna. Part 1
Nice video thanks for posting. What I see is your standard dipole only fed with ladder line, nothing special here except for the ladder line which induces darn near zero loss. I build my own using galvanized electric fencing wire available at any feed supply store and 1/2 inch pvc tubing from home depot, cut into 6inch sections with the wire fed thru holes drilled in each end (youtube has some posts). Tedious to built but super cheap and effective. I use a dipole cut for 160 m and she works very well down to 15 m. It looks like you parked down around quartzite. I miss that country, used to fly in there and land on the dirt n camp out every Christmass.
My father was a great believer in doublets fed with 600 ohm open wire feeders that we built ourselves with copper wire and wooden insulators that were boiled in paraffin. Looking back, it's a miracle we didn't burn the house down with the boiling wax on the stove. I understand the fumes were highly flammable. I think his favorite length for the doublet was 132 feet. That let him use it just about anywhere with the very wide range tuner he had for it.
My dad's tuner was built on a piece of 3/4 inch marine plywood, with a 3/8" thick sheet of bakelite on top of it. All the tuner components were on standoff insulators. Where the feedline was attached were six inch ceramic insulators. One day, Dad was tuning his KW rig, trying to get the antenna to match on a 5 MHz MARS frequency. This generated so much RF voltage that it made an arc from the top of the feedline insulator, through the bakelite, through the 3/4 inch plywood, and through the enamel on top of the freezer where the antenna tuner was sitting. Apparently, that point must have been a major voltage node. I remember the sound and the smell of ozone very well. You might want to watch that feedline length to avoid a similar situation.
An effective antenna! These charts are from DJ0IP website; these are optimum feedline lengths for 600 ohm feedline vs. doublet length. The circled lengths will tune all bands without the fireworks as described above!
If the 18 minute video was presented as text it would take less than 2 minutes to read. You do not want to use a 4:1 balun with this type of antenna system. The impedance at the balanced terminals of the balun is NOT the same as the characteristic impedance of the balanced feedline, it is the same as the FEEDPOINT IMPEDANCE OF THE ANTENNA, which varies greately depending upon which frequency band it is operating. Therefore, on those frequencies where the feedpoint impedance of the antenna is low, the 4:1 balun will transform that down to less than 50 ohms, which is MUCH more difficult to tune than an impedance that is above 50 ohms. It doesn't matter if it's a T network, L network, or Pi network tuner, it can handle a few thousand ohm impedance load much more easily than a load which is less than 50 ohms. The only time that you should use a 4:1 balun is if you KNOW FOR CERTAIN that the feedpoint impedance of the antenna is always 4 times the characteristic impedance of the equipment to which it is connected. An example of this would be the old classic TV antennas that were fed from a 300 ohm folded dipole and used a 4:1 balun to match the 75 ohm input of the TV set.
Confirms my own empirical findings that my past 141-foot open wire fed 135-foot dipole was on-the-money.
"You do not want to use a 4:1 balun with this type of antenna system." Are you recommending that a 1:1 ratio balun to transform unbalanced to balance line feed should be used instead? You never stated what should be implemented instead of the 4:1 balun. 73'
At the risk of sounding like Major Obvious, yes, a 1:1 CURRENT balun should be used, NOT a voltage balun. The 1:1 current balun functions as an RF choke on the outside of the coax shield, eliminating common mode current and RF radiation from the coaxial feedline.