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2m Dipole and Mast17 Oct '10, 19:09This weekend, I set myself a little project. Construct a dipole for use on 2m and an appropriate stand.
This weekend, I set myself a little project. Construct a dipole for use on 2m and an appropriate stand.
The project is pretty much complete apart from some minor adjustments and pictures are available on my Flickr page (see link in 'Related Links', or visit www.flickr.com/photos/m0gky/sets/72157625058766415/) showing the results.
The Aerial
The dipole, being a simple aerial, was very simple to construct. I decided to use my favourite of materials - 20mm PVC conduit tubing (available from B&Q), of which I had a number of offcuts from my Magnetic Loop project. All I needed for the dipole structure were two pieces of tubing, a T joint and a couple of straight-through joiners which I used to make the ends nice and tidy.
A BNC socket was fixed to the T joint allowing for the easy connection of RG58. To the BNC socket I soldered two lengths of wire, each a quarter-wavelength long (in approximation), resulting in a half-wavelength dipole. Once the soldering and drilling had been completed, it was simply a case of bolting the BNC socket in place, threading each wire into it's own piece of tubing and connecting the tubing to the T connector.
With the dipole constructed, I connected a length of RG58 between it and my 2m rig, balance the dipole between two curtain rails and gave it a try. Using an SWR meter and taking measurements at 144.025MHz, 145.000MHz and 145.975MHz I attempted to optimise the length of the dipole by trimming about 1cm of each piece of wire and then re-testing. I continued with this until I reached a SWR reading I was happy with.
The Mast and Stand
The next stage was to create a suitable stand permitting orientation of the dipole in either a horizontal or vertical polarisation and at any rotation I required. My solution was to create a small indoor mast with an H stand and guy strings.
The stand, like the dipole is constructed from 20mm PVC conduit tubing. Each leg has 1 meter of tubing, terminating in straight-through joiners with a hole drilled to accommodate the guy string. Half way along the leg, a T joiner is placed to allow a cross bar to be fixed, joining both legs together.
For the cross bar I used an offcut totalling about 40cm in length. Like the legs, this accommodates a T joiner at half it's length. This is where the central mast fits.
The mast is two pieces of tubing, each about 77cm long and connected with a straight through joiner. The split is so that the setup can be collapsed into a small easily portable package. At the top of the mast sits another joiner to which sits a short (20cm) piece of conduit with four holes drilled around it's circumference. This is where each of the four guy strings (one from each end of each leg) terminates. These guy strings are important to ensure the stability of the structure. 20mm tubing is not strong enough to support the aerial on it's own. It will bend out of shape without guys to keep it upright.
Next, the dipole can be connected directly to the mast using it's T joiner. This will allow it to be oriented in a horizontal polarisation only. Instead, I have attached a right-angle joiner. A very short piece of conduit extends horizontally from the joiner to which the dipole can then be connected. This allows orientation in either horizontal or vertical polarisation, but does put a bit of stress on the mast as it moves the centre of gravity. It's for this reason that the conduit extending between the right-angle joiner and the dipole must be as short as possible.
Still to do
The T joiner at the base of the mast, and the right-angle joiner both have removable covers. This means I can run a length of RG58 up through the centre of the mast and out of the top before connecting to the aerial. This will reduce strain on the mast as at the moment, the weight of the cable pulls down on the aerial which already moves the centre of gravity away from the mast as mentioned above. I need to order a BNC plug this run of cable but once that arrives, I can add this in.
As for the aerial, I'm considering the possibility of adding a director and/or reflector for improved directionality. I'm not sure if that will make it just too big for indoor use, but the advantage of using the tubing is that I can try it and if it doesn't work, I can just re-configure it back to a dipole.
Tags: aerial
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Flickr: 2m Dipole and Mastwww.flickr.com/photos/m0gky/sets/72157625058766415/
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Magnetic Loop Tweaks14 Apr '10, 15:39I've made some slight changes to my magnetic loop aerial. Nothing major, but useful none-the-less.
I've made some slight changes to my magnetic loop aerial. Nothing major, but useful none-the-less.
The original design, as copied from G4ILO, featured a T shaped mast with hooks on the left and right of the top bar to support a loop of RG-213 coax. The biggest problem I found with this was establishing the correct loop shape to create a good match. It was easy to squash or expand the loop which could destroy a previously good match.
To overcome this, I've redesigned the mast to a + shape. The same coax is used for the loop but it is now much more rigid than before meaning if I collapse the aerial and re-assemble it later, I am more likely to maintain a good SWR reading.
Other than that, I've now got a large run of RG58 coax to connect the rig to the aerial meaning positioning is much easier. I've also obtained a better fitting knob for the tuning capacitor. The one I was using was too big for the shaft making it off-centre, wobbly and more difficult to tune.
So what do I have still to do? Well G4ILO's design features an inbuilt SWR indicator which I might incorporate although it's not essential for me at the moment.
I'm still thinking about remote motorised control as well although gears seem to be more complicated than I thought. It is only the gears which are the problem though. The capacitor housing is large enough for a motor which will be connected to a long cable run (as long as the RG58) terminating in a small control box which can set next to the radio. All it needs to incorporate is a momentary push button control to activate the motor and perhaps a variable resistor to control the motor speed.
Tags: magnetic loop aerial
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09 Mar '10, 20:19Testing the Magnetic Loop08 Mar '10, 19:17Magnetic Loop Aerial
09 Mar '10, 20:19Testing the Magnetic Loop08 Mar '10, 19:17Magnetic Loop Aerial
Related Links
The G4ILO Wonder Loopwww.g4ilo.com/wonder-loop.html
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Testing the Magnetic Loop09 Mar '10, 20:19I've been further testing my magnetic loop aerial over the last 24 hours, but haven't made any changes to the setup as such.
I've been further testing my magnetic loop aerial over the last 24 hours, but haven't made any changes to the setup as such.
I did find out why, just before writing my blog yesterday, I couldn't get a good SWR reading despite doing so the day before. Rather foolishly I'd let the bare braid from one end of the RG-213 loop touch the outer contact of the phono plug to which the coupling loop connects to. Whoops! Once I'd realised that, getting a match didn't prove to difficult.
In actual fact, I think I may be getting used to tuning the aerial. I'm having very few problems setting up for 14.070MHz for PSK31 use. 7.035 doesn't match quite as well, but I can still get about 1.75:1 I'm hoping it's possible to get this better by tweaking that coupling loop - either fine tuning it's length or adjusting the shape.
Disappointingly though, I still haven't managed to make any contacts. I've been calling CQ using PSK31 on 20m and 40m, and also using SSB on 40m but to no avail. I'll keep trying but I'm not entirely sure how well my signal is getting out. The only indications I've had that I am transmitting are the appearance of markers on the PSKReporter map which indicate that a monitoring station has heard me. This has happened three times - in Sweden, Germany and Italy.
It does sound promising, but until I exchange some reports I'm really just speculating as to how well this aerial is working for transmission work.
Tags: magnetic loop aerial
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14 Apr '10, 15:39Magnetic Loop Tweaks08 Mar '10, 19:17Magnetic Loop Aerial
14 Apr '10, 15:39Magnetic Loop Tweaks08 Mar '10, 19:17Magnetic Loop Aerial
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Magnetic Loop Aerial08 Mar '10, 19:17
About 6 months ago, I visited the website of Julian, G4ILO who had detailed how to build what he has called a 'Wonder Loop' - a small indoor magnetic loop aerial. Following this visit I was inspired to build one myself.
As is my way, I've taken a little while to get around to building this but I have now completed my first build with moderate success.
I've used a 30-310pf (approx) tuning capacitor which I found on eBay for a few pounds, a loop made of RG-213 coax cable and a coupling loop made from some high current cable.
The capacitor is housed in a rather large plastic box (again an eBay purchase) using a couple of screws and a couple of cable ties to hold it in place. The terminals of the capacitor run to two terminal posts to which I can connect the main loop. The coupling loop is connected to a Phono socket, which runs directly to a BNC socket on the rear of the box. From this socket runs a patch lead to the transceiver.
I haven't been particularly precise in the construction - so imprecise in fact that when drilling holes for screws to secure the capacitor, I managed to mis-judge where on of the holes went. I ordered 2m of RG-213 for the main loop, and initially I used the entirety of this, only trimming it slightly during first tests. The coupling loop was one fifth of the size of the main loop, but since the trimming this is now slightly out of proportion.
The assembly of the aerial was completed yesterday after various trips to Maplin and B&Q to get parts, and initial tests are promising. As a receiving aerial I'm really quite happy. Sitting in the flat, it out performs my whip aerial when tuned correctly. Transmitting is a slightly different matter. Yesterday I managed to get a good match with SWR readings of around 1.5:1 or better on 40m and 20m and the PSKReporter website showed my CQ calls on PSK31 being received in Italy with 10w of power. Unfortunately today so far I can't seem to get a match on either band.
I'm experiencing an issue with the sharp tuning of the capacitor. I don't have a reduction drive so I'm having to be extremely precise with the tuning knob - possibly more precise than I am able to control. Another issue is the effect my hand has on the SWR reading. I've notice today that when tuning I can perhaps get a lowish reading but when moving my hand away, the reading worsens. Strangely I didn't notice this yesterday. I can't think of how to overcome this unless I can motorise the control. I'm not sure how possible that is.
My next steps I think are to look at the dimensions of the loop, and perhaps shorten it a bit. I will also try re-creating the coupling loop ensuring it is one fifth of the size of the main loop. I'd like to investigate the possibility of creating a reduction drive to allow for fine control but I need to source cogs and rods and figure out how to fit them into the existing design. If any readers have ideas of where to get such parts, please do leave a comment below.
Despite these initial set-backs, it's been fun to create what is my first home built aerial and to have it work albeit temporarily. Credit of course goes to G4ILO for his design, which I have pretty much copied, and write-up which was very helpful in aiding construction.
Tags: magnetic loop aerial
Comments: 1 | Related Entries: 2 | Related Links: 1
Comments: 1 | Related Entries: 2 | Related Links: 1
Related Entries
14 Apr '10, 15:39Magnetic Loop Tweaks09 Mar '10, 20:19Testing the Magnetic Loop
14 Apr '10, 15:39Magnetic Loop Tweaks09 Mar '10, 20:19Testing the Magnetic Loop
Related Links
The G4ILO Wonder Loopwww.g4ilo.com/wonder-loop.html
Comments
Matt de M6FNG 07 Jun '10, 19:36
Hi,I'm building one of these too. I have similar problems with the capacitor - My plans are to either make a reduction drive from plastic gears or add a small 25pf trimming capacitor for fine adjustment.I'm also replacing the coax with soft annealed copper tubing so I don't have to worry about keeping the form circular.Good luck and let me know how you get on!
Matt de M6FNG 07 Jun '10, 19:36
Hi,I'm building one of these too. I have similar problems with the capacitor - My plans are to either make a reduction drive from plastic gears or add a small 25pf trimming capacitor for fine adjustment.I'm also replacing the coax with soft annealed copper tubing so I don't have to worry about keeping the form circular.Good luck and let me know how you get on!