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Tuesday 21 July 2009

70cm colinear

Having moved my shack some time ago I lost the use of the 70cm antenna I had there so had to put up another one. Rather than move the old one I thought why not homebrew something new. A search through old PW found a suitable antenna. The diagrams were from PW July 2006, although the antenna design was originally from September 1992. G4XBY is the callsign of the July 2006 write up.

Click on the image to enlarge it.

What I built is the one in figure 1.

The antenna is a 5/8 over 5/8 colinear.

Materials should be 1.5m hard drawn copper wire, I used earth wire from 2.5mm domestic T&E although I believe thick coat hanger wire could also be used (as another is building his using it). You might have to play with lengths a little though.

The Figure 1 and fig 2 have the dimensions in mm. Figure 2 is a stacked set of 3 5/8 sections using phasing lines rather than phasing coils.

The two phasing coils are made by bending the wire at point B at 90 degrees and taping it hard up against a 12.5mm former (1/2" dowel was used). Figure 3 has a close up.

The matching section at the bottom is made like this:

Measure 178mm from point E, this is the very bottom of the matching section betwen G and H on fig 1. Make the bend as per the diagram for the bottom.

To set the antenna up, clip (I used croc clips) the centre of the coax (RG213) to the long side and the ground to the other, about 1/2 way up from the bottom. The length of the lead should be an odd number of half wavelengths at 70cm. If f = 434MHz then this is 346mm, applying 0.66 velocity factor for the coax and you arrive at 228mm. The coax lead for me was 7 times this length. If you don't do this all sorts of fun and games will ensue!

I tied the antenna to a bit of string and hung it from a tree in open air near the shack, about 6 feet AGL.

Then I looked at the antenna with a MFJ-269 and again with a MiniVNA. And adjusted the point of the match for the lowest VSWR, it was not that sharp but I found was just under half way up. NB you must make BOTH side equal from the bottom.

Then I added a more permanent coax cable by soldering it where the croc clips were, first sliding a 40mm OD pipe cap I had drilled a 10mm hole in over the coax (RG213), make sure you put it on the right way 1st attempt I didn't and had to reassemble the antenna, check the VSWR iss still low and then covered the joint with liquid electrical tape ( and after it set retested. I had a spare section of 4omm OD water over flow pipe after adding a gutter to the green house and now the cap makes sense and I slid the antenna into it. Was bit longer than needed but did not trim it. I added another screw cap at the top, at the bottom using the cap I had fed the coax through and then used pipe sealing cement (smells like old airfix glue) on both end caps, the screw caps can be undone if needed for an inspection. I added a little 3/4" x 6mm screw to secture in place with a blog of the liquid tape over it.

The 40mm OD pipe is WAY TOO big really and something like 20mm trunking would have been fine. For an antenna with more gain you can add sections, and make it longer. No idea what the gain is though and so far only managed a single 70cm contact with it to the local repeater to test it. GB3PY. The original antenna was a Diamond V2000 (2m, 70cm and 6m colinear) and I found this worked just as well.

You can also make the phasing sections as lines rather than coils too and figure 2 shows the details.

I built mine a bit differently but PW gave these suggestions for mounting.

Monday 20 July 2009

W3EDP Antenna

Well I posted a question if there were any long wire gurus on the GQRP list a while back to see if anyone else might recommend an alternative for my home qth. The issue is I seem to have a problem working sub 300 mile qso on 40m and 80m as I usually use a vertical on both bands as I have some interesting antenna restrictions.

The first is not really a restriction but I have agreed a 'no wires across the garden' rule which means that a permanent dipole (I have a 150-166 foot garden) is definitely out as I can neither put in a central support or have traps, balun, choke or ladder line in view. The reasons are multiple but the big one is neighbours. We are close to RAF Duxford (home of GB2IWM) and when the spitfires are flying ANYTHING and I mean absolutely ANYTHING is spotted in about 10 seconds. The only thing that passes is a fine single horizontal wire antenna but nothing must be supporting it. The W3EDP seems to be about the only thing that I can use.

Anyway I came across the W3EDP a while back and here are some web thoughts from QRP-L and other places:


I experimented extensively with the W3EDP wire antenna. The W3EDP, a variation of a true Zepp, is an interesting antenna, and one that I had high hopes for. It is described in Practical Wire Antennas by John D. Heys, G3BDQ; additional information on the W3EDP can be found in my Archives and Articles and in the article The FFD Antenna: A Field-Friendly Doublet, with Notes on Related Designs by Charlie Lofgren, W6JJZ.
The W3EDP consists of an 85' wire and a 17' wire that's sometimes called a "counterpoise". The counterpoise isn't connected for 80m or for 10m, but is connected for 15m, 20m, and 40m. The W3EDP is very easy to deploy--it needs only one elevated support, doesn't need a separate feedline, and packs up really small.
My initial trials of the W3EDP were using the MFJ-901B/HM-9 combination. I was able to successfully use the the W3EDP on 20m and 40m over the year that I experimented with it, on several operating events as well as on two trips away from home. Following the recommendations (article) of Charlie Lofgren, W6JJZ, I tried configuring my W3EDP such that the 17' wire was half of a parallel feedline, using 0.75" x 1.5" sheet styrene pieces as separators. However, when I tried this arrangement as an Inverted-L (using a sliding "button" insulator on the radiating element) from a cabin on Presque Isle, Michigan, I had trouble getting a good match on 20 and 80 meters. On subsequent trials with the 17' counterpoise lying on the ground it tuned easily on 20 and 40 meters, but I couldn't get a match on 80 meters. Clearly, the W3EDP/MFJ-901B antenna system was not the ideal all-band antenna system.
The arrival of the LDG Z-11 QRP Autotuner changed everything. The Z-11 easily tunes the W3EDP on 10, 15, 20, 40, and 80m through the homebrew 4:1 balun. I used this antenna from my billet at Rickenbacker Air National Guard Base during the month of June, 2003, suspending the 85' portion of the antenna between my third-floor window and the outside staircase of the neighboring building, allowing the 17' component to hang out the window. The tuner with balun easily tuned the antenna on all bands tried and QSOs were successfully made on 40m. I have also used the W3EDP/Z-11/balun combination in the field. I operated the 2003 E-PA QRP "TAC" Contest with the W3EDP/Z-11 antenna system, this time with the 85' portion extended in an inverted-L arrangement between two trees and the 17' component lying on the ground beneath the radiator. Again, the tuner easily tuned the antenna on all the bands tested and QSOs were successfully made on 20m and 40m despite poor band conditions. The W3EDP/Z-11 antenna system is a viable all-band antenna system. It's easy to deploy, tunes easily, and covers all the bands of interest.
I have 100' of teflon-coated wire wound onto an inexpensive plastic camping-style clothesline reel (photo). My plan with this wire antenna is to spool out as much as the available supports allow and operate it against the 17' W3EDP counterpoise.

An article by Bob Kellogg, AE4IC, from QRP-L about the W3EDP portable antenna. (07/12/1996)
An archive of emails between me and L.B. Cebik, W4RNL, comparing the W3EDP antenna and a 65' wire with counterpoise. (12/06/1996)
An article by Steven Weber, KD1JV, from QRP-L about the W3EDP antenna. (10/19/1997)
An article by Jay Coote, W6CJ, from QRP-L, about the W3EDP antenna. (10/20/1997)
An article by Keith, WB2VUO, from QRP-L, comparing his W3EDP to his 80/40m fan dipole. (03/01/1998)
An article by James Duffey, KK6MC/5, from QRP-L about the W3EDP and other long wire antennas. (03/03/1998)
An article by Charlie Lofgren, W6JJZ, from QRP-L analyzing the W3EDP antenna. (03/07/1998)
An archive of articles from QRP-L discussing the W3EDP, with comparisons to a dipole and a vertical. (01/06/2000)
An article by Monty Northrup, N5ESE, discussing his counterpoise system for use with the W3EDP antenna. (02/21/2005)
An email from W5TB discussing his experiences with the W3EDP and describing his technique for getting the wire in the trees. (09/01/2005

Friday 3 July 2009

PW transistor tester

Just opened post after being away fro few days /P operating (FT817 and MPX-100) from bicycle as cycled from Cambridge to Lincoln. PW was one of the magazines and I noted the transistor tester, I built an M-Cubed semiconductor tester kit bought at FDIM 2008 a while back and this looks interesting so will have a go at building it.

If like me you canot pass those stalls at rallies with lots of cheap transistors but not sure what they are beyond NPN or PNP or those odd power transistors for a PSU or PA project and cannot take a risk on a possible dud purchase then perhaps consider a cheap transistor tester like the one in this months PW, designed by M1GRY. I am going to buy a number of the jaycar cases for this and other test kit projects and going to order 10 or more which will mean they will be cheaper individually if anyone is going to buy/build one of these perhaps get in touch (UK only!) and I'll let you know costs. Rather than Maplin/Farnell/Jab etc I have found that fleabay regularly has some cheap 2x16 back light LCD displays too.Images to follow....

Design was by M1GRY.