Howdy,
When I retuned the antenna on my boat after changing its elevation (raised it about 5 feet), I noticed the tuning had changed, maybe due to distance from the counterpoise?
Howdy,
When I retuned the antenna on my boat after changing its elevation (raised it about 5 feet), I noticed the tuning had changed, maybe due to distance from the counterpoise?
Anyway, I now have a situation where another system I'm installing has the antenna at an elevation I can't reach, so tuning it in place isn't an option.
So if I tune it at a significantly lower elevation, how can I interpolate where it might resonate at the higher elevation?
A SWAG is simply a scientific wild A-- guess. The best you will be able to do with the system that is out of reach when mounted is to GUESS. Then listen at the outer extent of listenable signal. Then, re-tune and do it all over again. Yes, its a lot of work. My guess, that's why work is a four letter word. LOL.
You can do the initial tuning based on what other transmitters in the immediate area are tuned like. Its a starting point.
Or....you can lash a ladder to the mast, skitter up the ladder and do the deed. You might consider selling tickets ahead of time if you choose this option. You might draw a crowd. This follows the three most important words in paying the bills in radio: monetize, monetize, monetize.
In short, short of being taller, you are short of the ultimate solution to not enough personal physical altitude.
I hate SWAG. I even hate swag lamps. 😉
Anyway, are you saying it won't even be in the ball park if I tune it at a reachable height?
I'm now thinking about devising some sort of hinge or crane system.
Just standing next to the antenna will change the tuning. Not to mention that aluminum ladder lashed to the mast.
Of course, there is the venerable Sky Hook to hang the antenna on. Then you can use your imagination to tune the unit to wherever you want. And if you don't make a face while your tuning, no one will be the wiser.
Mashall Johnson, Sr.,
Let me see if I understand the Sky Hook idea.
O.K., first, ah...
What again?
I guess I'll just tether a weather balloon, like they did in WWII. Hafta get a helium tank and pull it down to refill it once in awhile, though 😉
I just wanna mention what a great help you guys have been on this thread, hee-hee ;D
The folks who work the low bands often use a v-e-r-y long extension to the tuning device, capacitor or internal secondary coil. I'm thinking the secondary coil might be overkill for what we're doing but adding a 360 pf tuning capacitor to the tuning circuit might do the trick
Add what ever length of non-conducting extension to the capacitor's tuning rod (fiberglass fishing rod blank, small diameter PVC pipe, plastic rod, etc.) to reach your tuning position to give you the ability to tune remotely.
Some hams use a power screwdriver to move a coil or change the winding spacing to fine tune, wiring a remote switch and tuning from a distance.
You could even add a simple micro-amp meter to measure current and monitor the meter with an inexpensive mini-video camera/web cam.
I think it may have been Phil and Ermie who pointed out that other devices weaken an already extremely weak signal. I think it will take careful calculation to figure out how much you'll have left when you finally get it to resonate. IOW, will losses be acceptable/worthwhile, or will it lose more than it gains?
I need to broadcast at 1630 on this end of town, and I wound the coil to be as close as I could get for a 3m antenna. Therefore there is no need to cover the whole bandwidth, so I think I could probably get away with much smaller range of capacitance and subsequent losses.
What say?
I haven't seen any readings identifying the use of a tuning capacitor as having the potential for weakening the signal.
Generally, the capacitor is described as a fine tuning device that will help the total assembly reach maximum resonance.
The range of the capacitor would help overcome any environmental issues and 360 pF isn't huge to begin with, so that's probably not a big concern.
The construction of the capacitor has been noted as important - the metal air-gap design has been described as superior to the compact models found in small AM radios.
Some additional readings below and much more here, just use the search bucket at the top right of the UI.
1610 KHz coil and capacitor tuned antenna
Old handbook on antenna construction with capacitors used in tuning sections
I used a ferrite rod on the end of a LONG
stick to tune a Part 15 AM loading coil.
Actually, it was plastic, not wood.
And it was a horizontal coil, not a vertical one.
Well - Ken - I guess this won't really help
you with your set-up. But maybe the idea
will help somebody.
Bruce, MICRO1690/1700
How about this (the one with the planetary reduction drive):
http://www.midnightscience.com/catalog5.html
?
A tuning capacitor added to a loading coil will reduce the Q of said loading coil. This is true whether the tuning capacitor is connected in series or in parallel with the coil. Some people find this to be surprising bacause the tuning capacitor itself has very little loss; but, nevertheless, the capacitor decreases the Q of the antenna circuit. For highest Q, the inductance of the loading coil should tune the antenna circuit to as close to resonance as possible. Then, with parallel tuning, a very small variable capacitor (only a few pF) should be used for tuning. For series tuning, a large stable, low loss, fixed capacitor (several hundred pFs) should be in parallel with a small variable capacitor.
Capacitive loading of an antenna actually increases the Q of an antenna circuit, unlike a capacitor with small dimensions, which decrease it. This is because the capacitive loading element (for example, a top hat, or a center hat) increases the displacement current of the antenna circuit, while a capacitor with small dimensions has no effect on the displacement current. Changing the position, shape, or orientation of a loading element on an antenna can be a means for fine tuning.
Thanks Ermi, that does make sense to me. The effort put into loading a short transmitting antenna is always an issue. A major problem with hats here will be wind.
1) What if I make a hat I can raise and lower along the active antenna length above the loading coil? That would seem easier to do. When it hits resonance peak, I can probably reach it with a tape measure, then bring down the assembly, lock everything into position and put it back up.
2) Is there a way to couple a tuning element, such as a length of 8 gauge copper wire, that wouldn't bleed power away from the main elements?
The thing that prevents me from getting to the antenna is an 8' glass mast ... originally a Shakespeare marine application.
If I take it out, I can reach the antenna. What will happen if I tune it to resonance, then put the mast back into the system, i.e., raising it up 8'?