I have a magnetic loop antenna for T/R with remote tuning. I would like to mount this in the attic horizontally. Will I loose much performance between mounting it horizontally verses vertical?
A small horizontal loop will have two sharp nulls: one pointing straight up and one pointing straight down.
- Help reduce losses in the soil directly beneath it
- Reduce the NVIS radiation directly overhead. On the lower bands, that could be a disadvantage if you want to work nearby stations, say, within a couple hundred miles.
- Lower the radiation angle
- Lower the gain at some angles and increase it at others
- However, you will lose the ability to null local sources of RFI by rotating it.
There is no issue with a horizontal small loop per se.
In fact, if you want a horizontally polarized antenna with equal gain in the horizontal plane, a horizontal small loop is a great way to do it. A horizontally polarized dipole by comparison would have two nulls off the ends of the antenna.
The nulls may or may not be useful for you. Nulls can be useful to exclude sources of interference. But if you can't rotate the antenna they can also exclude stations you want to contact.
Horizontal polarization is less effective for ground wave propagation. If your primary concern is DX contacts on 20 meters, this is irrelevant as these contacts happen through skywave paths where any polarization works equally well (or poorly, depending on conditions). If you are more interested in local rag-chewing on 80 meters you may have more of an issue, both due to the reduced propagation range of your own signal, and the likely case that most other people are using vertically polarized antennas.
Another possible side effect is that a horizontal transmit loop antenna in the attic might couple more strongly into the house's AC wiring below it than a vertical loop or monopole. The house wiring might or might not distort the antenna's pattern much; but you may have to be more careful with regards to RFI getting into household appliances and electronics.
The performance difference between horizontal and vertical is about the radiation pattern and what objects are near the antenna.
A horizontal loop really cannot be placed above metal surfaces because this will screw up the radiation pattern. Also the higher it is above ground the better.
A vertical loop can be above any surface no problem and it does not need to be high off the ground.
If there is no metal in your attic , attic insulation foil , or your roof , then you can mount your loop horizontally in the attic but it would be better if you could put it on the roof outside and get it up maybe 10 feet above the roof.
If it was me I would put a vertical loop 10 feet off the ground and at least 10 feet away from anything in the backyard , patio , whatever. If you aren't receiving anything you can go out in the yard and twist the pole and make it point a different direction.