Another long wire question - SW receive only

[6SN7WGTB]

I have a SONY ICF-SW7600, which I've recapped. It seems to perform well, and I have been experimenting with an external longwire antenna for SW. I have no specific band of interest, just general 2-20MHz.

I've about 25 yards of insulated wire about 10 feet off of the ground. Currently it enters via an open window and runs direct to the external antenna input. I know that this is sub-optimum, but as a proof of concept it's bringing in more stations under the right conditions. Yes, it also brings much more noise too.

Until now, the 'earth' has been internal to the building and a central heating pipe. Again I know not ideal, but one step at a time.

I have a cheap Howe ATU which can actually improve things. I've experimented with a VNA to see how it 'peaks' the desired frequency and done a table of ATU settings to freq. This too is no doubt sub-optimum as the VNA is 50? and the receiver probably not. But roughly-roughly it does seem to peak the signal tuning. (The antenna input schematic is attached).

Given that it does 'work' and does bring in more stations (I can get Radio Thailand in SE UK), I've moved the earth outside, added a coax to bring the antenna 'inside' to the radio. This significantly reduced the background noise - measured simply on headphone jack with RMS AC VM between stations.

But I'd like to do better, within the limitations of what I can install.

I have read what feels like a gazillion posts on aerials/antennae.

So, long intro, but my questions are:
1. Should I add a buried counterpoise? About what proportion of longwire length? I'm assuming I can use insulated 1.5mm^2. The 'end' can be either insulated, or not depending on what is better.
2. Should I be using a 9:1 or so unun?

On 1, I can probably add only one without becoming unpopular, and broadly 'underneath' the longwire. So not 15-20 fanning out radials!

A smallish detail, but the longwire drops to ground level where I have the unun or connection to the inbound coax. I cannot drop it at the far end and bring back via coax, which I believe would be better - yes, so the 'L. is at the less ideal end.

Thank you.

[paulsherwin]

Others may have a different view, but I'm not sure it makes sense to put a lot more effort into aerial design and matching with a 7600. You've already done the basic stuff. These are very competent radios, but they were never designed to be static comms receivers for SW use. If you're really serious about SW reception, it may be a better idea to concentrate on getting a proper comms receiver, either vintage or modern.

[GMB]

Yes, the normal complexities are to match a transmitter that is very picky about what it drives. Receivers can sometimes work better with crappy aerials especially if the aerial is more crappy at picking up local QRM than the actual signals.

Important point: if you bring an actual earth into a house then bear in mind, especially with PME supply, it can be a dangerous voltage during fault conditions relative to things in the house that are connected to mains safety earth (including pipes). Also, join them together and you risk huge current flowing.

[Bazz4CQJ]

I too own a 7600 and I think I agree with Paul that I'm not sure it is a set that I would try to use with more complex antenna systems, but maybe it suits your needs.

Is there anything on the net specifically about using 7600 with external aerials?

The term "counterpoise" is not too well defined, but I think the consensus is that a counterpoise is wire which runs at some distance above the surface of the earth and uses insulated wire, as distinct from a radial which is buried under the ground.

Have you looked in to the option of using a mag-loop aerial? Their advantage over a long-wire is that they are less prone to picking up the digital hash noise which is so prevalent now. They are compact and directional and do not require an earth. There's lots of info about them on the web and number of threads on them on this forum. The low-cost approach to magloops might well be looking up the "Wellgood" loop.

B

[Terry_VK5TM]

Long wires (or end fed) antennas don't normally need radials or counterpoises, such things are normally used with vertical aerials and just one is not sufficient.

An earth connection is beneficial, but keep in mind the above warning about none house connected earth systems

[dsergeant]

Your 25 yards of wire longwire is pretty identical to the longwire I have here, a tad higher but still quite low. I use this with my FT710 and work the world on 5W! Many people consider longwires as pretty poor but they can do remarkably well and are about as simple as you can get.

73 Dave G3YMC

[Radio Wrangler]

Long wire antennae tend to be rather high impedences over much of the frequency range you use them on. Receivers for these things tend to be fairly high input impedances.

So you can think of the antenna as looking like a voltage source, and the receiver as being voltage sensing.

But, voltage measurements still require you to have a reference point. Your DVM has TWO probes. So does your receiver. The receiver may be acting like a high value resistor, but that resistor still has two ends! With a long wire antenna, having a suitable earth is still critical.

The long wire setup involves only small earth currents, so you don't need lots of buried radials etc. If you don't even provide an earth connection, the mass of the radio will be the default and maybe any capacitance to mains. This will make the long wire and radio work from the point of receiving signals, but it will also receive noise and interference very well, too.

So, though you don't need much earthing arrangement, you do need it to be a quiet arrangement. It easily becomes a route in for every bit of grot radiated by every SMPS, eco-lightbulb and wall-wart for a mile around.

This has driven a lot of people towards loop-type 'magnetic' antennae which can be floated free of earth. which is another of their advantages in avoiding QRM.

David

[6SN7WGTB]

Very helpful chaps, thank you.

Let me respond to your collective comments:
1. Fully understand the earthing considerations. Have just spent some time 'correcting' a friend's SWA cabling in his garden, done by a 'qualified' electrician... Evidently a poorly-understood subject.
2. SONY did a powered active antenna for this receiver. I did procure one, but it turned out the FET amp in the head unit was (almost inevitably) blown. As it wasn't cheap, and was potted, I returned it as opposed to repairing it.
3. I use a loop aerial on AM, notably with my Bush DAC90. Only have to stand it adjacent, no connection, at the right angle, and tune in the secondary loop to get huge increase in signal strength.
4. Conclusion re radial (yes, sorry, not counterpoise) seems to be not worth bothering with. I may lay one on ground and see what subjective results it yields as this is easy to try.
5. FWIW I use a protector I have designed, between aerial and set, aside good practice given how much effort it took to repair!

Will play about with some loops more atuned to SW.

EDIT: just seen David's post. Yes the whole issue of interference was what started me looking to improve. My experience, as you read, was that NOT bringing the longwire into the house radically reduced noise.

EDIT2: if anyone can point me to a good SW loop design, or even proprietary product, much appreciated.