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Wednesday 20 September 2017

SDR and Heatmap

Not new but if you are either suffering EMC/RFI etc or just want to see how you or something else is doing you might like to note this software and comments.

I am sure many know it (dangerous assumption?). It will give you the option of scanning between a start and stop frequency and recording the location. Obviously, location depends on a good GPS RX and satellite visibility and if trying to scan the whole of the HF/VHF bands it is going to take a while but for a small 500KHz section of say 70cm it works fine so you can drive at more than a crawl. 

I used this along with a cheap GPS puck (ebay) and an RTL USB dongle (£10 from some rally), plugged into a laptop, mag mount (ebay for a 2m/70cm elcheapo one) on the roof, puck stuck to the windscreen and drove about and created a map of 70cm signals locally for an amateur
(who had an idea is was a pair of SRD TX 100m up on a pair of cranes already). The signals were blocking the output of a local repeater (20 or so miles away). After a few locals had used contest level beams from a distance to sort out the location to a few 10's of metres, I drove about the location and created a heatmap.

Anyway... it can help ID a noise source location, and/or where the signals are RX so might help some sort out interference. I have not tried but I suspect a suitable antenna on a car and a scan of one of the VDSL 'steps' from the recent VDSL survey and a drive about a village etc might be interesting.

I might write it up a bit more with images etc if anyone shows interest.
BTW yes you can use it at HF just the frequencies you start/stop on the scan HAVE to complete before you move. So be realistic if you are mobile driving.

Tuesday 5 September 2017

4SQ why homebrew you can buy all the bits :-)

One email was interesting.... I'll not reveal the sender.

In essence they asked... 4SQ why suggest anyone homebrew you can buy all the bits :-) I guess you can for a price. So lets just take this as an example... probably one of the more 'popular' hybrid controllers, this one even has a send the signal to all 4 a the same time option (expensive to add that bit I am sure!)

£598.99   Inc VAT

The 75 ohm matching cables £ 420 Inc VAT

You then have the actual 4 GP verticals, coax feeder to the control box, a 12-14V power supply that will switch the relays.  Very very conservatively add another £600 Inc VAT

That gives you a system that will cost about c£1500

Amusing to say the least.

Ok it is capable of KW+ and no construction or figuring out required. Well not much anyway, use a tape measure to layout the antennas and then screw a few PL259/SO239 together.

I'm going to see what a homebrew system for 40 might actually cost for qrp fans. Decent enclosures, so IP56 or better outside, powder coated Ali for the shack, masts and all cables and connectors included and pre-measured as an option as well. I will probably do the same for one using KW capable parts for those DX trips, and another for UK home and club use 100W/400W. I wonder if anyone would be interested? Contact me if you are as would be interesting to see what the interest is.

Might even see about using a 5th element as per the NCJ article a while back.

Questions following GQRP talk

Telford Rally / GQRP talk have had a few emails all basically asking the same info.

Ground screw:

The screw was bought (with a load of others I keep using/reusing) from a garden centre years ago.
Yes I HAVE posted whole boxes full to DX before hand so I do not have to carry and to make sure they get there and antennas fit but never had a problem with flying. I'd put in hold luggage though, security are bound to confiscate a 2 foot spike from hand luggage! A trip is not going to fail if the bag goes missing with this in it (maybe if the radio was in it too but I have never lost a bag yet), you can usually find something else to use when there. Think beach parasole base or car show room mount.

All info I have on them I posted a long time ago in 2012:
There are all sort of clones out there, YOUR local garden centre will probably do them if not then google for it... one choice was
If don't need to fly in a plane or not too worried about weight consider getting a galvanized steel one as the plastic ones do break if ground too stony.

The alternative if operating from the car might be a drive on mount and I noted Sotabeams/Richard had some at Telford and I really liked them.


The original was made back in 2013. Seriously nothing rocket science about it. I needed a set of dipoles for multiple bands, one a time that was very very light weight so it WILL have all sorts of issues about line losses etc.
If not too concerned with trap losses, which are minor to be honest, I would make it up using SOTAbeams TRAPS as will be MUCH easier and no playing with having to pull it down and reconnect etc when changing bands.
or for more power
I like the idea of 100% homebrew but honestly by the time you have mucked about with making a PCB getting the tracks right sourcing the components you might as well use the ones from SOTABEAMS.

DO NOT use shrink wrap, in fact NEVER use shrink wrap on antenna parts that will be exposed to the weathger, especially near the sea as you will have created a lovely place for moist salty air to corrode stuff you will not see until too late. Use liquid tape to water proof the parts/joints.

4 SQ bits:
see follow the links on the pages especially ZL2iFB you'll find a spreadsheet in there.

4 SQ is is NOT my work but a classic and I rely on the publications ofthat of ON4UN, TK5EP, G4IFB (now ZL2IFB). I am pretty sure there is something more recent but NCJ covered options a while back including add a central 5th element (4el were enough for me) which can add ANOTHER 20dB. The gain is on RX signals not TX. You 100% CAN BUILD for other bands just scale it.

For those that want to eek that bit more out see: 

Phased verticals:

Monday 4 September 2017

What is the F/B RX ratio of the 4SQ you use?

Interesting question asked at GQRP rally talk.

What is the F/B RX ratio of the 4SQ you use?

Thanks for the question at talk. Never been asked that one before.

It is all a bit theoretical but F/B ratio > 20-25dB should be dead easy to obtain and 30dB is not impossible. Some have claimed > 45dB if you get the phasing and separation right but unless lucky I think that is pushing it a bit and when DX you don't quite have that long to tinker and have to make do with something close.

When as i mentioned in the talk when DX I can tell you you absolutely CAN null out one direction or another and it is possible to have a qso with a station in one direction (qrp) whilst in another there is another station at 59+

I am pretty sure there is something more recent but NCJ covered options a while back including add a central 5th element (I never got it to work and 4el were enough for me)

A 4 sq should get you SOME gain in one or more direction on TX but it is the RX F/B gain you want.

An extra note:

There is SOME TX gain but I never advise on any existing (think of it like a 1/4 wave vertical). I don't know if I even said that in the talk.. The RX gain F/B is what blew me away first time I heard it (6W/M1KTA station on 40m I could qso with JA clear as a bell (when EU played nice!) and null out NA/VK and visa versa... of course if some LID is on same beam heading and it is kinda wide you are a tad out of luck).

I found one man 'could' take a 4 sq, put it up, use it, take it down and it is all possible. sub 10kg for the lot, and not a £1500 price tag too 

BTW you do NOT need an estate ... first one I made at home was a 17m version (everything scales!) using T50-6 so it will fit in a domestic garden. qrp you can just point to point the whole controller too, don't need a PCB, I think my first was in an altoid tin. The box I waved in the talk (just like the ones you will see on this blog) was a 1KW capable thing. The 'HEAVY BIT' is the 1/4 wave matching lines from controller/switch box to the separate antennas as they HAVE to be made right and all 4 antennas HAVE to be identical and resonant where you want to operate. BTW I don't believe in tuners, why waste 50% of the RF
in the tuner?
Use RG179 (not RG174)  for the matching sections if build this qrp but watch out for cable being trod on. Why I started using WHITE SAT/TV coax as more visible (albeit heavier).