Here is a view from the ferrite ring choke looking towards the end of the antenna.
Here is a view from the ferrite ring choke looking towards the end of the antenna.
Starting at the right side is the choke. Half way to the tower is a strain relief where the coax ends and the center conductor continues.
At the end the antenna hangs from a spring which hangs from the guy wire. There is a string attached. The string was used to pull the antenna up the guy wire and is tied off at the tower.
Question About the Tower
In MRAM’s picture we see his 55-foot tower, and my question has to do with such towers.
I assume that tower, a metal structure, is bonded to the earth at its base, and is intended not to be an antenna, but to hold antennas raised up high.
But for long wave operation under part 15 we need a 50-foot antenna that is isolated from ground. Is it possible to order a tower installation that is isolated from ground, so that the tower itself becomes an antenna?
If not, how could a plain tower be employed to hold a proper 50-foot antenna isolated not only from ground, but also from the proximity of the tower itself?
50 foot isolated antenna
Carl – Several manufacturers make what you’re looking for, including MFJ Enterprises in Starkville, MS. They make a 64 foot 11 section telescopic aluminum mast that becomes an isolated antenna if you also buy their “base mount”. Rather expensive, tho…mast is $200…base mount is $70. I bought their mast several years ago — using it to support my ground plane for FM. I also bought the base mount, but never used it. So, if you can find a suitable mast with a 2″ OD bottom section, I have the base mount to isolate it from the ground with. Sell it to you for half price, ($35) brand new still in the box! If you need to buy MFJ’s mast, the stock number is MFJ-1965. The base mount number is MFJ-1900.
Lemme know!
P.S. — If you use the mast as an antenna only, and don’t mount anything on it, you shouldn’t need any guy wires for your 50 foot stick. Easy!
Under Study
Thank you rlkocher for the details.
I will find out about local codes to find out if I can mount a 50′ tower without inviting weasels.
If a 50′ tower can stand without guy wires, what keeps it from blowing over in a 15-mph wind?
Shunt Fed Grounded Tower
I have read articles describing a way to load a grounded tower. Seems similar to a J-Pole antenna.
In most cases anything on top of the tower becomes part of a “top hat” load.
I’ll have to look through my ARRL books to see if I can find that again.
Tower staying power
“If a 50′ tower can stand without guy wires, what keeps it from blowing over in a 15 mph wind?”
A lot of it depends on how heavy the top is compared to the bottom. The mast I referenced is a telescopic model — bottom section is 2″ OD, top section is .625″ OD. So the bottom is much heavier than the top. That stablilizes it, so it just sways in the wind.
You CANNOT however, mount an antenna on this mast at 50′, without using guy wires. (20′ would be ok) The weight on the top would cause it to buckle in the middle, even with just a breeze blowing!
Disputed Dipole
The End Fed Dipole is not a dipole according to a post in the Revews of Reviews thread where the subject came up.
Bringing back this Post From the Past we can see a real-life example of the End-Fed Dispute-Pole Antenna.
Not Your Traditional Dipole But A Dipole None The Less…
Yes, a traditional dipole by definition has two legs and is fed at the center.
In that regard the vertical “coaxial” antenna has two legs and is center fed but the coax comes up through the inside of the cold side of the antenna (a metal tube) and connects to the upper hot side and the lower cold side at the center. These were popular where horizontal radials were not desired. Not your traditional dipole.
In a manner of speaking the coaxial antenna is end fed as the feedline enters the bottom end of the antenna.
Taking it a little further the “end fed” dipole has two legs. One leg, the hot side, is a single conductor. The other leg, cold side, is of equal length but is continuous coax shield. The cold “end” of the dipole is developed by the high impedance presented by the ferite choke. Effectively the outside of the coax shield is an open circuit at that point.
Wait a minute, if it’s effectively an open circuit how is it part of the active element? Well just as for a regular dipole the RF flows along the inside of the shield. Where it terminates at the center fed dipole RF can flow back along the outside of the shield. This is why a balun is typically used at the feed point of a center fed dipole so the RF does not flow back the outside of the shield.
So think about it… For the end fed dipole the RF flows up the inside of the shield to the center of the antenna where the shield physically ends. Then the RF flows back down the outside of the shield until it encounters the high impedance of the ferite choke. So, effectively the RF is fed to the center of the end fed dipole much like the traditional dipole and the end result is the same as the traditional dipole.