Dr. Ulrich Rohde: Electrically Short Antennas

So that raises the question: what can we, IC-7300 owners do to ensure we can keep operating this rig in the coming years?
I was thinking about bringing the receiver path out to two connectors, to be able to add an external preselector. I happen to have a nice 6 x 250pF varco that would make up for a nice 6-stage preselector.

 

Yes, a good preselector always helps.
The difficulty is what is called the in-band intermodulation, like a few Khz spacing.
For 40 or even 20 m some 10 or more Khz. Crystal filter at the RX input will rescue you.

 

There are several answers to this problem, what can be done at the receiver side is essentially this;

1. Use the attenuator function at all times when the full sensitivity is not required
   
   
2. Use external preselectors, or in extreme cases crystal filters, in the RX signal path
   
   
3. Use narrow-band antennas with tuned couplers
   
   
4. Hope that the HF broadcasters do not come back in their former glory
   
   
5. Keep a healthy distance to amateur neighbours
   
   
6. Hope that as few as possible amateurs nearby become active on HF (there are far too many amateurs anyway)

73/
Karl-Arne
SM0AOM

 

Purchased a copy just now

 

You really should apologize; that would be the classy thing to do.

 

Positions 4 to 6 are both correct and funny...

 

The WBAP transmitter is 2.6 miles from my house. It's a 50kw transmitter on 820 AM. The signal is 60 over S9 here and no overload issues.

My TS440 wouldn't handle it with the attenuator off but my SDR stuff doesn't have any issues.

 

There is enough spacing for the correct filter to act.In the 40 m band and 5 Khz broadcasts transmitters the world will be different.

 

Maybe I don't quite understand. Eventhough I am in close proximity to this AM broadcast station I do not have issues listening to it or other AM stations on adjacent frequencies. It's hard for me to believe broadcasters on 40m could render SDR useless some day. Seems like the beginning of a Y2K scare.

 

To just have one strong unwanted signal to suppress is a quite benign situation for an HF receiver, compared to the situation when there are many.


The above plot, from a mid-80s paper by Gott&Laycock may give some idea of the HF spectrum congestion of this era.

In the "Prime spectrum estate" region of 5 to 10 MHz there could be found many tens of S9+50 signals from the broadcasters more or less at the same time.

An "arms race" between the propaganda broadcasters was raging and it was possible at times to get over one volt peak out of a resonant dipole.

This, and the even worse problem of co-location on "small platforms" have made me and other professionals involved in HF systems engineering very aware of the actual limitations of different receiver architectures.

73/
Karl-Arne
SM0AOM

 

Thanks for the clarification. I'll cross that bridge if we get there.

I don't say this to be rude, but with so many professionals on amateur radio forums it gets hard for common people like me to know what to believe.

 

A good beginning is to actually listen to those that know better.
This was the way how I myself "learned the trade" almost 40 years ago.

73/
Karl-Arne
SM0AOM

 

The description is correct, I think it is difficult for the average amateur to expect the strong multi tone situation today.In the USA I find broadcast stations at night from 7.2 to 7.3 Mhz that make the overload red signal in the Icom 761o become alive and while the tracking filter is not good enough in the CW band , the attenuator does help.

The current architecture needs to be refined.

 

Very true. These days it takes a good bit of filtering to figure out who knows better and who is pretending to know better. Plagiarism runs rampant on the internet and the blind lead the blind.

What you said about the 40m broadcast problem makes more sense now. What you are describing is an overload and intermod problem. When I first skimmed over the thread it seemed as if some were saying SDR would soon be useless on all bands and the end was near.

 

When asked "SDR or analogue/hybrid" I usually reply:
"SDR solves a lot of problems, but also creates some new".

If the decline in broadcast HF usage continues, the "broadcast problem" may soon not be anything to consider more seriously.

However, the "close neighbour" or "co-location" problem will be with us for the foreseeable future.

I had the good fortune of learning the basics in the "Pre-Internet" times, and to have teachers, colleagues and superiors who knew what they were talking about.

In the mid-80s I befriended master RF designers SM5QA(SK) and SM5HP who let me "sit by their feet" and learn about advanced HF receiver and transmitter techniques.

I hope to be able to give some guidance and inspiration for the few young HF engineers that join us.

73/
Karl-Arne
SM0AOM