I have seen where hams use a 160 meter on a vehicle held by a massive mag mount. So here's a question, since the magnetic mount couples the signal to the vehicle as a capacitor, is the vehicle actually connected to the antenna? If this is the case and there is no connection, there would be no actual ground lead.
It would seem that the capacitive coupling at 1600 kHz between the antenna feed point and the vehicle body through a magnetic mounting scheme would be minimal. The mobile HF ham antennas are usually bumper mounted and the feedline ground is connected to the chassis or bumper for grounding.
At VHF, the capacitive reactance is low enough that effective RF grounding to the vehicle body is obtained with a mag. mount.
Neil
Are right on. At low HF frequencies when running mobile, a good ground along with bonding is essential for reliable mobile communications. Running on 160 meters mobile is similar to running Part15 execpt for the power. By bonding everything together enables the vehicle to capacitivly couple to earth a little more effectivly. Not so critical on say 2 meters where the vehicle alone does the job.
WDCX AM1610 Part 15
John
Owner-Operator-Chief Engineer-Program Manager
http://home.earthlink.net/~wdcx
Do I see a new type of mounting scheme for xmtr's? (Rangemaster in mind)
I envision a magnet mount on the bottom and an rf block on the actual ground wire, Supply cables, Ect..
(Gotta be a big one to make sure it doesn't blow away, Ya know..)
Seriously.. How much magnet mount area would it take to couple a xmtr to a ground? (Per frequency range) This might be a good thing..
Or, Possibly a "Gamma Match" type mount..?
Hmmm...
~Don
Don,
Seriously.. How much magnet mount area would it take to couple a xmtr to a ground? (Per frequency range) This might be a good thing..
If you want to pursue this:
Start by assuming what capacitive coupling (Xc) is needed. From this
Xc = 1/(6.28*freq*C) solve for C:
C = 1/(6.28*freq*Xc) or just pick a reasonable C.
From C = eA/d we can calculate the area A:
A = Cd/e.
Now we would need to assume or know the value of the dielectric coefficient of the paint and the paint thickness. Not a very hard calculation, but lots of assumptions.
I didn't run any numbers but my off the top of my head guess is the area for 1600 kHz will be bigger than the typical vehicle rooftop. (This is what is known as guessineering).
Neil
Ok, maybe I'm seeing this incorrectly.. But for mobile operation, wouldn't the capacitive connection to the ground be talking about the metal body of the vehicle as one "plate" and the surface of the planet as the second "plate"? Pretty much like any mobile operation, since the tires are insulators? I wouldn't think the magnetic mount would be an advantage except for not drilling holes in the body of the vehicle and usually an electrical ground to the metal body exists anyway via the vehicle's battery and electrical system ground?
Daniel
The vehicle would function as a ground plane to return induced currents to the feed at the base of the antenna. At BCB wavelengths the feedline ground to the chassis would probably serve this purpose and customary practice is to ground the feedline to the chassis at the base of the antenna. Given the large dimensions and loading coils used for HF mag. mounts may be an interesting idea but hardly practical.
As we go to higher frequencies, especially VHF, the feedline becomes a significant portion of the wavelength and the best and most practical approach is to provide a RF ground at the antenna base either by direct ohmic connection or capacitive coupling. For example, my 2 meter (144 MHz) mag. mount has a base 2 inches in diameter and that is sufficient.
Neil
How about using a building with a metal roof instead of a vehicle. Most metal roofs are sloped and have ridges and channels to guide rain water. Consider this, mount a flat metal plate slightly eleveated above the roof with a solid electrical ground connection. Then place a dialectric on top of the plate and then place another plate on top of this. This would make a huge capacitor, of sorts. The question then becomes how large would this "capacitor" need to be? Would several large plates be an option to reduce the overall size? Could someone run the numbers?
How about just connecting the transmitter or feedline ground to the metal roof directly?
Neil
I luv optimistic views..
I believe enough capacitance could be derived from a gamma match set up. It takes "X" amount of capacitance to pass "X" frequency.. Doesen't it?
by MRAM 1500 kHz
Actually, oversimplifying, it would be more like 1/X increase in capacity for X increase in frequency. Higher frequency would require less capacitance for the same reactance.