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March 28th 18, 12:08 AM
posted to rec.radio.amateur.antenna
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Antennas That Don't Need Grounding For An Apt.
On Tuesday, March 27, 2018 at 11:46:25 AM UTC-7, Dave Platt wrote:
I am taking my ham test in two weeks and I just found out you have to ground your equipment. I live in a apt without copper, water pipes. I have heard a dipole antenna would work. Any comments. There are three issues here with respect to grounding. One is "safety" grounding - grounding and static-discharge of outdoor antennas, to deal with lighting and with dry-air static buildup. This sort of grounding should be done outside the building structure, using a proper lighting arrestor and heavy ground wire. It's more urgent in some parts of the country than others, but it always a good idea. A second is AC safety grounding - using proper three-prong AC outlets (with building grounds) for any equipment that has a three-prong plug on its power cord. Don't use "cheater" plugs which bypass the ground - it could create a shock hazard. Proper grounding of the equipment chassis also helps avoid RF burns, if any RF on the antenna "feeds back" into the equipment chassis due to an antenna fault. The third is using ground as part of the antenna system (as, for example, HF monopoles do). This seems to be mostly what you were asking about. Yes, dipole antennas can operate without a connection to earth-ground, as one half of the dipole provides a current path for the "ground" side of the RF signal. In order to keep RF off of the outside of the feedline (where it might travel back to the equipment chassis) it's a good idea to install a balun of some sort at the connection between the dipole antenna and the feedline. With a standard coax feedline and a typical dipole you'd want a 1:1 balun; if you're using a folded dipole you'd want a 4:1 balun; if you're using an off-center-fed dipole you'd want something else (I think these usually use 4:1 baluns as well). A full-wavelength horizontal loop is another sort of balanced antenna that doesn't require a ground connection. A third choice is a "small" or "magnetic" loop antenna. These have the advantage of small size (relatively speaking) and they can usually be tuned to cover several of the amateur HF bands. They have disadvantages - relatively low electrical efficiency (high losses), a narrow operating bandwidth (they must be re-tuned every time you change frequencies more than slightly), and high RF voltages and currents that can present a shock or RF-exposure hazard. The good ones aren't cheap, as they need an adjustable capacitor which can handle those high RF voltages without arcing and the high RF currents without burning up due to electrical losses. One of these might be your only good choice if you're working HF from an apartment and don't have the ability to install a big outdoor antenna. As for operation on VHF (2 meters and so forth) - the issues are a lot simpler in practice. There are numerous 2-meter antennas which don't require a ground connection to work - they're either dipoles, or "end fed" antennas, or they have their own radials which serve as a ground plane. You can make a very simple and effective "ground plane" 2-meter antenna yourself, from little more than an SO-239 socket and five pieces of stiff copper wire. This is electrically quite similar to a simple vertical dipole - its radiation pattern and electrical efficiency are almost the same as a center-fed vertical dipole - and it's quite apartment-and-balcony friendly. http://www.hamuniverse.com/2metergp.html Thanks. |
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