2015-11-04

Active antenna

I have finally got my active antenna connected. This is an old Dressler ARA-60 that officially covers 100kHz (50kHz ?) to 60MHz with a substantial gain, and a decent dynamic range.

First things first. Using this antenna while transmitting does not work very well. No surprise there I have my 10m vertical about 5m from the active antenna, so it is, of course heavily overloaded when transmitting 100W on 10. All true signals simply disappear. No, I am not disappointed, I was expecting that, even with my 3W CW, that I use mostly on the HF bands with a dipole, overloads the amplifier.
Similarly, the medium wave broadcast signals tend to generate IMD in the receivers on frequencies below 500kHz. At least, that is how I interpret all the apparent spurious carriers on those low frequencies.
Also, I am not yet using the original power inserter (connectors have to be provided), so I am using a simplified power inserter with a high value inductor. This works fine on MF/LF, but, unsurprisingly the inductor appears to have some resonant frequencies, making the gain vary over the (large) frequency range. I suspect that when I get the correct power inserter ready that will change.

I am splitting the signal from the antenna into ~>49MHz and ~<30MHz, using a commercially available diplexer. The >49MHz signal is then entered into a broadband power splitter (3 outputs), going to scanner receivers. One goes to a multimode scanner, listening to beacons, meteor scatter from TV transmitters, SSB, CW etc. and the other to an FM scanner receiver. The <30MHz goes via a 4-way satellite TV signal splitter to (at the moment) a single HF receiver covering  ~0 - 32 MHz. Things work nicely, and I have good signals on all HF bands - noise level is rather high on some frequencies. On 600m the signals through the splitter are still quite sufficient, although there seems to be some IMD present. I expect to use this setup later for QRSS and WSPR monitoring, most likely with simple home made receivers.

It will probably be a good idea to split the frequencies further, e.g. with a low pass/high pass filter centered around 500kHz, in order to remove the MW band signals from the receiver for the lower frequencies.

Now, just listening, the antenna provided a nice surprise. The spec tells me that the antenna is good up to 60MHz, but the "local" beacon on 70 MHz (about 40 - 50km away) comes in nicely. The receiver set up for the 70MHz FM calling frequency of 70.450MHz suddenly came to life, and a station about 180km away appeared on frequency via tropospheric propagation (tropo).

All in all this is a nice generally applicable receiving antenna. The limitations are clear to me, but it presents a good compromise until I can improve the antenna system, and is good for general LF/MF/HF/6m/4m monitoring.

No comments:

Post a Comment