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Best rooftop antenna?

Apr 11th, 12:33

WA4VEG

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
My HoA will allow me an HF antenna on our building's flat roof so long as it isn't visible from ground on property. The bldg is steel & concrete with a membrane/gravel on roof surface which is about 75 feet AGL. I've experimented with a 75 meter dipole about 6 feet above roof with very limited performance. I'd appreciate any HF antenna recommendations for this location.

73, Bob
wa4veg@arrl.net
Apr 13th, 07:34

W1VT

Super Moderator

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Reinforced concrete acts as a lossy sponge, absorbing your signal. It is also possible to have noisy HVAC equipment that degrades reception. More separation will help. The demo version of EZNEC can model this--just substitute the parameters for soil/ground with those of concrete.

Using NEC/4 and (0.015,4.5) as the "soil parameters." It would be classed as "medium soil."
6ft -2.5 dBi gain
12 ft 2.3 dBi gain
20 ft 5.3 dBi gain. The model suggests you can gain a substantial 8 dB gain by just getting your antenna 14 feet higher.

An amplifier may help with the lossy antenna. Going from 100 to 600 watts is about the same improvement as raising the antenna from 6 to 20 feet. Though most people need an amplifier on 75M SSB running a normal antenna. CW and digital work better if you can't make your signal stronger.

An small RX loop can help if you orient it to null out a local noise source.

Zak Lau W1VT
ARRL Senior Lab Engineer

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