ATU for SWL?

[Yorkie]

I finally installed my Snowdonia 9:1 unun and long wire antenna today hooked up to my Eton g3.

Quite an improvement to reception, but want to know if I can get more out of it.

So, would a ATU make a difference?

Also, the unun has a grounding point that I haven't used, would using it make a difference?


Regards
Yorkie

[wind in the antenna]

An ATU can be very useful if you are not using a resonant antenna or if you are using a resonant antenna outside its optimal band. An ATU helps by bettering the power transfer between the antenna and receiver and compensating for this even though the antenna is out of match. It also by consequence has the advantage of being a band pass filter and thus attenuates signals outside the tuned frequency.
So yes, an ATU is very useful in an SWL context.

The good news is that an ATU for RX is far cheaper than a TX ATU due to the power handling component costs involved when handling higher signal levels. Obviously avoid anything aimed at the CB market as this is inevitably going to involve some really nasty design and build values.

[26TM944]

For a receiver you want a Pre-selector not an ATU. ATU's are more for matching antennas to transmitters (or vice-versa). Most people using an RX with a random antenna would not notice much change in reception if the SWR was 10:1 or 1:1.

As you're feeding the RX with a balun and 50ohm cable then the match at the RX is already pretty near 50ohm and you wont get much from an ATU.

However a Pre-selector will offer you a tuneable front end - which allows for some RX to antenna matching BUT will improve the IMD of the radio dramatically by removing out of band signals (or at least attenuating them).

Most older radios had a tuneable front end - this was a Pre-selector - the ATU was never designed to "filter" the front end - however they do in a lot of cases cross over each others work. A very high Q ATU will peak RX - you can hear this on a lot of modern radios as the hash gets louder as the ATU peaks the noise. However - the Pre-selector will offer a better job - as you're not really trying to "match" the antenna and feed line - as you've already done that.

Pop to a local radio shop (if you can) and try out a Pre-selector and an ATU on a RX - you'll see the difference. Note a lot of the commercial Pre-selectors have built in pre amps etc - unless they are very lossy or used with a very short antenna you should not need to turn that on.

I have an RF Systems P3 - which is probably the best pre-selector about for MW/SWL use - completely passive - and works great in front of a SDR or a conventional RX.

73

Steve

[crusty]

I finally installed my Snowdonia 9:1 unun and long wire antenna today hooked up to my Eton g3.

Quite an improvement to reception, but want to know if I can get more out of it.

So, would a ATU make a difference?

Also, the unun has a grounding point that I haven't used, would using it make a difference?


Regards
Yorkie

The UnUn should smooth out any high impedance spikes so an ATU isn't strictly needed, but can help with intermod if the ATU includes switchable bandpass filters. As mentioned a preselctor is more beneficial (Lowe PR-150 very good if you can find one for sensible money), or an noise canceling unit like the MJF-1025/1026. However, a big improvement you could make first is to ground the UnUn to an external earth.

[Farty]

I'm going the UnUn route myself soon, and an extra 10 feet of long wire elevation - much higher and I'll be needing a chinook to install it!

[ChickenMadras]

Probably worth saying that RX only ATU's (or QRP TX ATU's) are cheap to make as you do not need components to handle large amounts of TX power.

They are a good first project to make if you want to get into homebrewing kit.

[Yorkie]

Thanks again for the advice.

So a pre-selector would be the best choice, any recomendations as it looks like the RF Systems P3 was discontinued some time ago. Or even some build plans to build a decent one?

Also, advice on grounding, should I be driving spikes into the ground? If so, how deep, how many etc?

Regards
Yorkie

[crusty]

Thanks again for the advice.

So a pre-selector would be the best choice, any recomendations as it looks like the RF Systems P3 was discontinued some time ago. Or even some build plans to build a decent one?

Also, advice on grounding, should I be driving spikes into the ground? If so, how deep, how many etc?

Regards
Yorkie

For a buy it now SWL preselector there's not much choice. The MFJ-956 seems popular (but overpriced IMO). Hunting for a s/h unit like those mentioned, or posting a wanted ad on TM1 might be worthwhile.

Grounding is a subject of much debate. One six foot rod is better than nothing, more rods are better. Multiple rods should be spaced at 5 foot intervals and connected in series. Though I'd question the need for elaborate ground networks for SWL. Keep connection from UnUn to ground as short as possible. I found best results when installed as follows:


Just one thought to consider: The G3 wasn't designed for high performance on big wire antennas (not knocking the G3, great portable unit), it might be wise to consider a s/h dedicated HF receiver in the long run. ATB.

PS. If you go driving earth rods be sure you know where your water/electric/gas services run.

[mikecharlie]

the MFJ-956 will work, but it is WAY overpriced in my opinion in what is actually in it.......but it will give a reasonable tuning peak and is still a usefull add on.

Value for money wise, the Watson Wpr-2 Preselector, looks as if its the same exact type of passive preselector( I dont own one), with the added benefit of a attenuator switch on the front panel( which the MFJ doent have) ,and thats its a more sensible price than the 956.

WPR-2 only seems to be available on Airband-Online though ( they are a good company BTW)


(Nice diagram crusty........!!!!! )

[Yorkie]

OK, I'm confused why to fit the unun at the far end as wouldn't I get signal loss with a long run of coax?

To explain my setup better, I have edited that picture to show my current setup as below. The unun box is screwed to the wall on my house.



Have I got it setup wrong? I'm happy to change it if people think I can get better results.

Thanks again
Yorkie

[Farty]

That's fine, though in an ideal world you'd use Crusty's layout and get the feedpoint as far away from the house and its QRM as possible.

[mikecharlie]

Well....this will work basically as well as crusty's diagram, but what is more likely to happen is that youl'll maybe pick up more elecrical interference noises from devices in your home....because the active antenna wire and 9;1 feedpoint is very close to your house.

Having the 9:1 balun at the far end AWAY from the house, with a long coax lead- in as far as possible will help to cut down any interference from the house stuff , and is a more convenient earthing connection also.

Having a longish run of coax, wont really have much loss at HF frequencies the way it would with UHF etc.......and you should have the benefit of less QRM on reception.

[Yorkie]

OK, thanks again. I think I will turn this antenna around then as an issue for me would be fitting an earth point. Even if I ran one down the wall of the house, a conservatory is below it and a lot of paved areas so would end up with a long dog legged routed earth wire.

Plus I would gain about 4m on the long wire dropping downwards to the unun close to the floor at the far end. I can't drop the long wire straight at the house end due to conservatory.

Good point about QRM, on the same wall I have a 1m motorised satellite dish so maybe the LNB and disque signals may cause QRM?

Looks like a need a drum of coax though, would there be an issue burying it in the lawn on the run to the house?

Oh, another thing to consider, when/if I get my act together, will this setup be OK for transmitting when I get my foundation?

Regards
Yorkie

[tomos]

Are these not made any longer .. A good package ?

http://radiopics.com/Accessories/Global_ATU-2000.htm

[crusty]

Burying the coax is ideal, but check if coax is suitable, some don't do well in soil (the cheap stuff usually).

If the UnUn you have is rated for TX, you're good. Likely you'll adjust the wire length to suite the Ham bands at some point (what will be the length of your wire run approx?).