Constructing a 2m colinear Part 1

Hi all,

My recent 6 summit SOTA trip contained a few QSOs on 2m. All bar one of mine were using a Wouxun 2/70 HT with a 1/4 wave antenna on 2m. This is convenient, but is really only one step up from the stock antenna supplied with the unit. The antenna uses the radio and the operator’s arm as a (in)effective counterpoise, imposing significant loss to the system, well over 10dB. It’s time for a bit of gain.

Why a colinear?

I had a look at N1HFX’s colinear page among others. There are various types of colinear out there, some are based on a jpole, while this is a series of alternating half wave sections. The design from N1HFX is about a permanent installation, but I wanted something that could be used on a SOTA activation. There is a debate about whether to use a beam or a high gain vertical. The beam can be changed from horizontal to vertical and back by changing the mounting of the beam, but here in Victoria, most chasers would have vertical orientation. Horizontal does come out on field day contests, but aside from that vertical does seem the go.

A second consideration is that chasers might come from a variety of different directions. A beam is going to have a main lobe in one direction, with a series of nulls in others. This is good for some use, especially where either the direction of the wanted station is known, or there is some local source that is wanted to be nulled out. On most SOTA summits, there is not large local sources – obviously there are exceptions – but also radios such as the FT-817 do have good rejection of these. The chaser direction is not always known, and can vary quite widely.

For these reasons, I decided to go for a colinear. I also plan to use it as my vertical antenna alongside a Quadruple Quad for horizontal on the John Moyle MFD, which is the only VHF/UHF contest that I spent much time thinking about. Even for that contest, I generally think about running a station, taking advantage of my equipment optimisation for SOTA by going to places that require foot or non-car access to the summit.

Squid Pole and Coax choice

In the last few months, I have been often using a 7m (actually 6.8m) squid pole to provide a centre mount for a 20/40m end fed or the structure for a tunable vertical antenna for the HF bands. I began to think that this squid pole could provide a structural platform for a 2m colinear. The N1HFX design calls for 8 half-wave segments. I have been using LMR195 for a lot of my coax cables. This is a RG-58 replacement with better loss performance. One effect of this is that it has a higher cable velocity, measured with my MiniVNA Pro at around .825 One issue with these higher velocity ratios is the cable needs to be physically longer, compared to RG58. So, I needed to actually use RG58, rather than a drop in replacement for this colinear. I bought some from www.rfsupplier.com/

The MiniVNA Pro measured the RG58 cable from rfsupplier at 0.66, so this makes for a shorter setup than LMR195. It will actually fit on the Squid Pole with 8 half wave segments, rather than having to go to a 4 segment version. This should make for a nice antenna.

The first job is to cut up 8 lengths of RG58. I settled on a centre frequency of 145MHz. This is to provide for 144.1 SSB and for 145.5 and 146.5FM simplex not being too far away. We’ll see how that goes. Each length needed to be 300/145 /2 *.66 = 682mm. Add an extra 8mm for the overlap needed between two segments as per N1HFX’s design. Here’s a pic of the 8 segments cut and both ends stripped:

8 RG58/U segments cut and stripped

Then, I needed to join up the 8 segments as per the N1HFX design. Here is a look at one of the joins. A reminder that any image on this blog can be clicked for a better view:

Coax join to swap centre conduct and braid

It was a little tricky ensuring that there were no shorts between the two centre connectors. A stray bit of braid could some down (and sometimes did) to effect a short. I needed to continually check for this while soldering each connection. This is a gotcha with this design. I did pump on a good amount of solder to provide a good join and taped up with electrical tape to ensure the braid stayed away, and also to prevent water getting in.

The top end of the colinear needs a 1/4 wave segment, but only connected to the centre conductor. I grabbed some old RG6 cable that I had already stripped away the braid from for this purpose. The bottom end needs a 1/4 wave sleeve connected to the input coax braid. I used some aluminium tubing for this. Finally, there are dreaded common mode currents with a colinear. N1HFX has a choke at 1/2 wave down the tube for this. Obviously they cannot be put at the feed point because of the 1/4 wave sleeve, so 1/2 wave down the feed-line has to do. I would much rather a few turns. I could have gone for more turns on some PVC tubing, creating what is essentially an Ugly Balun. I went for three small toroids with 3 turns. This should provide a very high impedance to a common current, and at 1/2 wave from the feedpoint, this high impedance will be presented to the antenna.

Here is a pic of the completed coaxial colinear:

Coaxial colinear for 2m

Now, all I need to do is to carry this and the squid pole on site, mount the squid pole and the end of the colinear on it, and I have a BNC connector to go in the FT-817 or any similar style radio. I’ll test on a SOTA activation in a few weeks.

Regards,
Wayne Merry VK3WAM

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3 comments on “Constructing a 2m colinear Part 1

  1. […] my building of a colinear antenna for 2m, it was time to try it out, and what better than a SOTA activation. I picked three summits […]

  2. […] This is a continuation of Constructing a 2m colinear Part 1. […]

  3. […] have described my efforts in building a colinear for 2m over several posts. The base unmatched colinear was sort-of ok at 146.5, but not very good at […]

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