Mag Mount attic antenna

[KE0GXN]

I have a few questions for anyone who can answer and for anyone willing to give tips/pointers/advice. I have a mag mount antenna base that I want to use to put an antenna I have in my attic. I do not want to fix up an outdoor antenna since I will only be in this house for about a year longer. I decided, based off some help from other hams, that an antenna in the attic is a decent compromise. It is still mostly effective, and I wont have to deal with too many headaches for the short time it is up.

Anyways, it is a mag mount antenna and I know that it needs some kind of metal for the ground. There is a huge metal vent pipe that goes from my attic to the basement, and I am wondering if this will suffice for the ground. It is nearly 20 feet long, but the bands I am going to use are uhf and vhf. Will this negativity affect the performance, or will this help.

Any pointers or advice is helpful in this!

Thanks, Katt

I used a large pizza pan with a mag mount and a HT set-up for several weeks, until I was able to purchase an outdoor antenna and a mobile base. Worked good for me.

 

[sdu219]

If you use a cookie sheet, pizza pan or a pie plate and it has "NON STICK" surface. Take a sander or sandpaper to it and get right down to the bare metal. I use one on a transmit application and it works great. Just screwed a metal pizza pan to a window sill and put the mag mount on it and it talks gang busters R-9 everyplace it needs to talk.

 

[prcguy]

I don't think sanding to the bare metal will buy you much of anything. Hard grounding the antenna feed point to the cookie sheet yes, but a mag mount has a plastic coating over the magnet and it usually goes on a painted car, so that's not much different than a few mils of non stick surface.

If you use a cookie sheet, pizza pan or a pie plate and it has "NON STICK" surface. Take a sander or sandpaper to it and get right down to the bare metal. I use one on a transmit application and it works great. Just screwed a metal pizza pan to a window sill and put the mag mount on it and it talks gang busters R-9 everyplace it needs to talk.

 

[jonwienke]

If you use a cookie sheet, pizza pan or a pie plate and it has "NON STICK" surface. Take a sander or sandpaper to it and get right down to the bare metal.

With most mag mount antennas, that's a complete waste of time. The mag mount base acts as the plate of a capacitor, and has an insulated coating that also reduces the likelighood of scratching the paint on your car. The tuning of the antenna expects the RF ground to be coupled through the base capacitor, and if you short that capacitor out by sanding everything down to bare metal (the coating /paint on the mount surface and the base of the antenna) you can detune the antenna.

 

[prcguy]

Not so much with a VHF/UHF antenna. You can take most of them tuned on a permanent mount then put them on a mag mount and the tuning is about the same. The capacitance through most mag mounts is adequate to couple the ground for VHF/UHF but not HF/CB. For that I have made a magnetic capacitor plate by coating an 8 1/2 X 11" sheet of refrigerator magnet material with copper foil and bonding that to an HF mag mount with a very short wide strap. The tuning on a 40 or 80m Hamstick or CB antenna is nearly identical to the same antenna on a hard grounded mount. Take the capacitor magnet of the HF mag mount antenna and everything gets squirrelly.

Diamond Antenna makes a tiny version of my magnetic capacitor that is too small and has ground lead that is way to long but they seem to be selling in the $30 range. You can make a much better one for about half that. Diamond Antenna MAT50 7 to 50mhz Band Magnet Earth Sheet for sale online | eBay

With most mag mount antennas, that's a complete waste of time. The mag mount base acts as the plate of a capacitor, and has an insulated coating that also reduces the likelighood of scratching the paint on your car. The tuning of the antenna expects the RF ground to be coupled through the base capacitor, and if you short that capacitor out by sanding everything down to bare metal (the coating /paint on the mount surface and the base of the antenna) you can detune the antenna.

 

[jonwienke]

The tuning on a 40 or 80m Hamstick or CB antenna is nearly identical to the same antenna on a hard grounded mount. Take the capacitor magnet of the HF mag mount antenna and everything gets squirrelly.

I've had a similar experience with a CB antenna. But I understand your point about VHF/UHF. The higher the frequency, the less different the capacitor is from a direct connection.