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First experiment trying QRP CW kit with an end fed wire antenna? (3 replies)

M7RPH
10 months ago
M7RPH 10 months ago

Hi,

I've recently passed my foundation exam (thanks to EssexHam and Pete M0PSX for the preparation).

I have just built a very small 3 watt (QRP Pixie) kit which operates using a 7.023MHz crystal and some very limited tuning).

I want to use it to start teaching myself a bit about practical HF antennas and I've been looking at end fed wires with a counterpoise. I would appreciate a sanity check before I begin playing.

My plan is to sit at the kitchen table with my laptop and SDR dongle and next to it my little CW transceiver.

My initial idea was to run a length of coax from this, out of the cat flap and onto the garden fence. From the shield of the coax I want to drop down a counterpoise and from the core will run a 1/2 wavelength length of wire. I'm beginning to understand that if I want to use a length of coax then I would need an unun to prevent the coax radiating (this does not present a major problem except currently I don't have the materials to hand to make an unun).

If I am impatient and eager to get started right away (I am) and I was to simply connect the 'long' length of wire to the inner part of a BNC connector and the counterpoise to the outer, run the long wire out of the cat flap and the counterpoise along the kitchen floor in the same direction would that work?

In a half wave wire antenna, the current will be at a maximum at the mid point, 1/4 wavelength along which means it should radiate most of its energy outside (rather than slightly warming up my laptop case and kitchen table)?

Apologies if all this is elementary, just by typing this message some of the details have become clearer to me, nevertheless I would appreciate any feedback anyone thinks is pertinent. 

I also wanted an excuse to say hello. (I still haven't managed to make myself heard on any nets with my little Baofeng, but that's another project)

Steve M7RPH

 

Peter M0PWX (2E0PWX)
10 months ago
Peter M0PWX (2E0PWX) 10 months ago

Hi Steve

i thought at foundation level you were able to transmit using kits only commercial equipment, as its considered you don't have sufficient knowledge and or ability to test and maintain the kit to ensure out of band emissions are within limits, and ensure you main transmission frequency stays within the band

as to the antenna i have a home made fan dipole attached to my fence, very similar to what you are proposing, mine being a balanced dipole i only need a current UNUN / choke at the feed point to help control common mode issues on the braid

one thing i have found is i followed certain information on the net about using PTFE insulated wire for the UNUN, the down side of this is the wire is thick and you can't get many turns on a FT240 core, a better solution i have since found is use the thicker gas PTFE tape to wrap the core and then use enamelled copper wire (the ptfe tape protect's it from damage by the sharp edges of the ferrite)

another thing you will find is balanced antenna's like a dipole are far less noisy than long wires, i had a long wire initially and my noise floor in my radio was around -60db to -70db according to the display, when i swapped to the fan dipole this dropped to around -90db to -100db and i can resolve far more stations

but at the end of the day its all about experimenting, try your proposed solution, then modify it and see what you see in sensitivity, resolvable stations and the distance you can reach

also note that you will get further at various times of day so vary when you listen and transmit

also look at a nano VNA, these are great bits of kit to check antenna's for SWR and many other measurements and less than £30 on ebay for the older version or about £40 for the newer model, and a free program for windows called "VNASaver" makes it much easier to use, look at youtube for more on these

M7RPH
10 months ago
M7RPH 10 months ago

Hi Peter,

Thank you for your reply. I've been hunting around for a solution to an SWR meter but had not come across these, this device and VNA saver really blows the ceiling off my expectations of what is possible on a budget. Thank you also for your advice on FT240, PTFE tape and enamelled wire, I will take this on board start planning a concealed fan dipole across the garden fence.

Regarding the restriction of the foundation license, I had read that although I am not allowed to build transmitting circuits of my own design, it was permissible to build a commercially obtained kit.

After reading your comment I have done some more reading and I understand now that there is a IR-2028 clause. I have found a lot of discussion online about this (this thread I thought was particularly interesting: https://groups.io/g/RSGB-Workshop/topic/69201180#2584).

My own interpretation had been that the QRP Pixie kit is a commercially available kit and it is from an established and published circuit that has been used in RSGB workshops which, even if it is not certified as IR-2028 compliant (as no such thing seems to exist) it appears to follow the regulations of this clause.

If I am mistaken here then I will stop working with this kit. I can still concentrate on building a suitable antenna system for the SDR dongle (which if connected to a raspberry pi in the garden) will keep me out of trouble until I can take the exam for intermediate.

 

Kind regards :-)

 

Peter M0PWX (2E0PWX)
10 months ago
Peter M0PWX (2E0PWX) 10 months ago

Hi Steve,

my understanding was if it was a kit then a foundation level op couldn't use it for TX, there was a lot of discussion on the essexham.group.io forum about what constituted a kit under the clause, and its such a grey area open to interpretation though

if you are a member of the RSGB there was also an SWR / power meter based on a teensy microcontroller with a 2.8" touch screen to build for about £70, it was in the november copy of radcom pages 38-42

also if you haven't long passed your foundation look at the intermediate as so much of its the same, just in a little more detail and not in my mind a massive step up

 

look at joining https://gm6dx.thinkific.com/ its a free online intermediate course similar to essexham's foundation, that and the exam book got me through mine 6 weeks after i passed foundation with 42 out of 46 right :) , then you are clear of any "grey areas" around the QRP Pixie :)

also look at https://www.ebay.co.uk/itm/Tiny-Spectrum-Analyzer-TinySA-2-8inch-Screen-100khz-960mhz-V0-3-Touch-Control/ its a spectrum analyser for £50 same size as the NanoVNA useful for checking harmonics etc of a kit

lots of little things like this around 

have a look at QRP Labs Kits (qrp-labs.com)

note you can't transmit through the VNA or spectrum analyser

73

pe0PWXeter

2

 

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