Posts Tagged ‘472khz’

472kHz preamp – a good idea?

The FT817 is a bit low in sensitivity on 472kHz so I was expecting a decent improvement with a preamp. To my surprise I am copying no more stations. In fact I am hearing just as many without as with! The result has surprised me. The rig sensitivity is definitely better with the preamp and stations are stronger but it is far from clear cut whether it is worth using it in reality. At the moment my 472kHz transverter does not have the preamp.

Maybe the FT817 sensitivity is good enough. In marginal cases (and these seem few) it probably helps, but most of the time it seems fine without.

I fancy some 472kHz WSPR!

It is ages since I was active on 472kHz, so I might fire up the FT817 and transverter to see if anyone can spot my 5mW ERP or I can spot others. I shall use the earth-electrode “antenna” and go QRT on 6m to free up the FT817.

It is summertime, so noise levels will be higher, making my weak signal harder to copy than in the winter. It will take me about 5 minutes to swap bands and reset everything. The FT817 has to be set to 500mW and split mode so I receive directly on the 472kHz band but TX on the 80m band into the transverter.

If you copy me on 472kHz WSPR please report it.

UPDATE 2009z:  Well, that was a good start! I just switched to MF and received 2 spots by G8HUH in IO81mg (250km) in daylight. I shall now leave 474.2kHz WSPR running overnight to see what else happens. With 5mW (maybe less) ERP I am quite happy with his -26/-27dB S/N reports.

UPDATE 2126z:  So far G8HUH (250km) has spotted me 10 times this evening. I have spotted no-one and no-one else has spotted me, as yet.

Over 1000km on 472kHz TX with earth-electrodes

Last night LA4ANA (1047km) copied my 5mW ERP from the earth-electrode “antenna” on 472kHz WSPR. This is my best DX yet on the non-antenna, nearly 300km further than my best previous TX record.  I was also spotted by DL4YHF (556km) for the first time on the earth-electrode “antenna”.   My PC decided to crash some time after midnight last night, so I missed any DX overnight. These spots were during the evening period.

In the last 2 weeks alone I have been spotted by 28 different stations when I’ve been using 5mW ERP from the earth-electrode “antenna” on 472kHz WSPR.  Let no-one say 472kHz is out because they cannot fit an antenna! If my signal can span over 1000km with WSPR then you can surely do better. I make no claims for the earth-electrode “antenna” but if you are unable to erect a proper antenna give it a go.

I have still to try it at this QTH on 136kHz but at my old QTH I managed to be spotted at 250km with very low ERP.

472kHz WSPR using earth-electrode antenna

As an experiment this afternoon and evening I am using the (short baseline) earth-electrode antenna. I am being copied by G0LRD (25km) and G3ZJO (79km) so far. It is now 1620z. Initial results suggest G0LRD is getting me very slightly better on the earth electrodes, whereas G3ZJO is getting me slightly weaker. This is comparing the earth-electrode “antenna” with the HF /VHF antennas with strapped feeders tuned against mains earth. In the latter case I resonate the antenna with a 110mm diameter coil with many taps.

One end of the earth-electrode is tied to mains earth. The “far” earth is a 1m long earth rod driven into the soil. The connection to the far electrode is 32 x 0.16mm PVC covered wire running along the fence at a height of about 2m. This wire dog-legs and is about 15m long. The spacing between the “far” electrode and mains ground in the shack is about 12m max. At the old QTH the baseline was more like 20m.

I think the earth-electrode antenna acts a bit like a loop so best results tend to be in the line of the loop. It also means there is not a huge difference between the two systems. You could say they are both equally bad, but the earth-electrode system needs no matching coil. I have optimised the resistive match using a 3C90 toroid (step up) between the transverter and earth-electrode antenna. At the old QTH it looked close to 50 ohms so the toroid was not needed.

When fitter, I’d like to try an earth-electrode system with a much bigger baseline.

There is no doubt that my 472kHz antennas can be much improved. The question is, “how seriously do I want to try?”

ERP calculation on 472kHz

In an attempt to establish my ERP on 472kHz, I have crunched some numbers. I think it is error free, but no doubt someone will soon tell me!

With a 1:46 step up toroid, I am measuring the DC volts rectified across a 470 ohm secondary load resistor using a small 50uA meter in series with 240k ohms (120k twice) and a series silicon diode. This circuit is straight out of the “LF Today” book available from the RSGB bookshop.

DC current = 30uA
DC voltage across 470ohm secondary load  = 0.6 + (240 x 10E3 x 30 x 10E-6)  = 7.8V
DC voltage (rms) across 470 ohm = 7.8/1.414 = 5.52Vrms
I secondary = 5.52 * 10E3/470 = 11.7mA
I primary =  46 * Is = 46 * 11.7mA = 0.54A

Rrad = 160 * pi^^2 * Heff^^2/lambda^^2 = 160 * 3.14 * 3.14 * 2 *2/(630 * 630) =0.0159 ohms

P(erp) = I^^2 * R = 0.54* 0.54 * 0.0159 W = 4.6mW

My original guesstimate of 5mW ERP is not far out!

Clearly getting the antenna much higher would help enormously as ERP is proportional to Heff^^2. All one can really be sure of is the order of magnitude. I am running low milliwatts.

472kHz – improvements?

Using the strapped feeder to my Par 10/20/40m as my 472kHz antenna rather than my 2m/70cm feeder, the antenna current has nearly doubled with the antenna current meter now showing 30-33uA whereas it was 18uA before. This suggests ERP has increased by around 4-5dB (ERP proportional to I^^2), but we’ll see later. The Par antenna itself is lower than the 2m/70cm antenna and runs horizontally to a tree. Average height is around 3-4m AGL. The losses may be greater.

The picture shows the 472kHz coil used (a ferrite rod would be fine but since moving it is “lost”), the 472kHz homebrew transverter (left) and the 3C90 matching coil between transverter and antenna (towards centre, LHS of coil). The small meter measures antenna current and registered 18uA last night and 30-33uA now with the Par antenna with strapped feeders.

UPDATE 1420z: G8HUH (250km) is copying in daylight a full 8dB S/N better than last night. Encouraging.

UPDATE 1448z:  M0PPP (182km) seems to be coming in stronger than last night too and in broad daylight.

UPDATE 1700z:  M0PPP  is now spotting me at 1456z, 1538z and 1620z i.e. in daylight.   He was not copying me at all last night, so I think my signal IS stronger.


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