RE: LF: MF powerful Beacon in Poland?

[Rik Strobbe <rik.strobbe@fys.kuleuven.be>]

Hello Richard, Luis, all,

 

a horizontal dipole at low height (versus wavelength) is in theory an inefficient antenna. See http://www.strobbe.eu/on7yd/136ant/#HorDipole.

But:

1. "Low height" is less than a few % of the wavelength. At 7% of the wavelength the radiation resistance is already 10 Ohm. On 136kHz this 7% is an unrealistic 150m, on 475kHz it is "only" 44m! At a height of 100m (as suggested by Richard) the radiation resistance on 475kHz will rise to 40 Ohm!

2. The better (more conductive) the soil the worse a low horizontal dipole will be. Or the other way arround: the worse the soil the better the antenna! So in certain conditions a horizontal dipole at a height of 20m or more might be working pretty well on 475kHz.

 

As often there is only one way to know: try it ;-)

 

73, Rik  ON7YD - OR7T

 

---

Van: owner-rsgb_lf_group@blacksheep.org [owner-rsgb_lf_group@blacksheep.org] namens VIGILANT Luis Fernández [luis@vigilant.es]
Verzonden: zaterdag 30 januari 2016 20:21
Aan: rsgb_lf_group@blacksheep.org
Onderwerp: Re: LF: MF powerful Beacon in Poland?

Hi Richard

I'm probably the only one in MF transmitting with an antenna installed in a 80m high building (26 floors)

By Benidorm standars this is also a small tower. But I'm lucky to have access to top terrace to grow my

antenna farm ;-)

I agree with the rest. Any beacon signal is wellcome and useful. CW, QRSS or digital all counts

and helps to increase activity and rise the interest of other hams

>I live in a 15 floor building, on top of which there is a 30 meter mast, and my building - by Warsaw >standards - is "small". (I set a QRP world record in 2005 from such a building on 3.5mhz)

Do you have access to this 30m mast ? Would it be possible to hang a vertical slope wire from top

of the mast to top terrace on floor 15th ?

>Thus to hang a dipole on such a building would not be expensive and to mount a 1w TX at the dipole >center point would make beacon construction simple.

A dipole is not an option on MF. If the power limitation is 1w radiated power, you will have to use

many watts on Tx to get 1w radiated, specially with a low efficient antenna

What I'm using here with great success is a 11m vertical with two 6m "arms" as top load

Then in the base of the antenna a coil about 3mH to get it in resonance. The ground line of PA

runs to ground using a water pipe inside the building. This is not "ground" but the extension of

the vertical antenna. So the antena is really a vertical formed by  80m + feed + coil + 11m + topload

If you build something similar at your building tower it should provide similar results

My QRP tests ended by injecting 1mW to the antenna and then I got my signal decoded using

Opera32 at 300Km by EB3FRN. All this on 630m band

Your mileage may vary ....as US guys say ;-)

73 de Luis

EA5DOM