VK1JA’s 6el 2m Beam

Last Thursday for the VK1 2m FM Lunchtime Net we had Jayson (VK1JA) run a comparison between his normal vertical and then a switch over to a beam he had modified to tune up on to the higher end of the 2m band. He found definite improvements (even with the the beam just ocky strapped to the pagola and connected with RG-58 – the vertical is connected with RG-213) and he has sent through his NEC model to share.
 
So here is for anyone who is interested, and I've also generated and attached the pattern for peoples viewing pleasure. ūüôā
 
(Jayson has also sent me through the model for a dual band – 70cm/2m – beam he is working on and I'll post that up shortly too.)

2m_6el_single_band_final

Using Unequal Lengths for a Phasing Harness

Alright, maybe not the clearest title for a post, but hey. ūüôā

Anyway, in my previous post about what I plan to do with my phasing harness for stacking my planned WA5VJB "Cheap Yagis" I was deliberately attempting to keep things simple so as to keep the post short. However it has been brought to my attention that the over simplification may possibly lead people astray, so let me correct something and provide you with some more information to ponder.

In my previous post I said:

"(But keeping in mind that the lengths have to be the SAME odd multiple – so both 1/4wl or both 3/4wl – otherwise the phase will be out and potentially cancel or something.)"

Now I made mention here two things:
1) The two lengths must be the SAME; and
2) If they're not the SAME they'll cancel each other because they're out of phase.

This was an over simplification so let me make these two elaborations:
1) The two lengths can be DIFFERENT – as long as they're both an odd multiple of 1/4wl; and
2) If they're different that can result in the two beams being 180deg out of phase – for example if 1/4wl was used for one and 3/4wl used for the other.

If you do chose to go with a setup that results with them being 180deg out of phase and you have them both orientated the same way then they will cancel each other out – kind of defeats the purpose hey! But there is a simple solution. You rotate (along the axis of the boom) the beam 180deg. This will then obviously put it back in phase and maximum benefits will be realised.

In fact, VK1OD has done that with his setup detailed here:

With mine though, I plan to keep it as simple as possible (as it's my first attempt) so I'm going to use identical lengths and identical beams. ūüôā

But on this note, I will post up another post on the basic theory of how the 1/4wl section (also¬†colloquially¬†known as a "Q-Section") works as an impedance transformer; as well as the formula for working out what Q-Section you need (as in what impedance) and also the¬†significance¬†of a 1/2wl section (explaining thereby why we specifically want odd multiples). And all that is based on phasing, so stay tuned. But that probably wont be today, however hopefully one day soonish. ūüôā

Have a good week-end!!

Before Work 2m SSB DX – The Cheap Yagi Proves a Success

This morning for the second time I headed up  Red Hill (QF44NP) for some before work 2m SSB DX. This time though I had a slightly more significant antenna setup.

Last time I just used my half wave dipole on top of a 2m mast mounted up on my bullbar. This time though I had a 3m mast and my 4 element WA5VJB "Cheap Yagi".

Plus I also went with a more minimal power supply. Normally I lug around my ~30Ah Gel Cell battery, but this time I thought I'd try and go minimal with a 1.3Ah 12V SLA (see photos). This little guy worked fine, however I did notice that now and then the low power indicator (for 2.5W) was flashing – so perhaps the power output was sometimes down near 2.5W. I do have a 4Ah SLA which was the first one I grabbed for portable operation – but it's at least twice the weight – so I might run some tests with that over the week-end.

Anyway, with the new antenna and mast I setup on my bonnet so that I could easily 'peak' the antenna while watching the S meter on the FT-817. This worked a treat, as the beam has a workable F/B ratio so for the weaker VK3 signals I could really aim it as required.

Over all the session was a success with the most morning interstate contacts I've had to date (one – VK3II – with rather significant QSB – aircraft enhancement). So here's the log:

Call – Recv'd QS(T) – Sent QS(T) – Notes

VK1BG – 5/9 – 5/8 – Ian
VK2BX – 5/7 – 5/8 – Gerowa
VK2GKA – 5/7 – 5/8 – Karl @ Penrose
VK1KW – 5/9+ – 5/9+ – Rob
VK3II – 4-5/1 – 5/5-7 – Jim
VK3AJN – I could hear him, he could not hear me

A few others were heard too, but not strong enough to try and contact – especially with < 4W.

So all good fun, and really keen to get up there again before work one day soon. (Oh, and the maiden run for the Cheap Yagi was a definite success Рand only took moments to erect as opposed to my 10el 2m beam.)

Photos:
1 – The antenna on the mast mounted to the bull bar;
2 – A shot of the setup on my bonnet (with the nice little 1.2Ah SLA battery); and
3 – A shot up at the antenna from the ground.

Photo_1Photo_2Photo_3

Today’s Lunchtime Net from Black Mountain

Photo

Today saw another great VK1 2m FM Lunchtime Net with 9 attendees Рeven with me running about 10 minutes late. This time I hosted the net from the top of Black Mountain as it was quicker to get to from where I was running late from.

We had a great range of stations from a range of locations. We had one station right in the middle of Civic in Garema Place and another out on the Hume Highway between Bookham and Yass (heading towards Yass). We even had another station do a comparison between their vertical antenna a 6el 2m beam which proved very interesting.

Thanks again one and all for participating, and thanks even more so for sitting by and waiting when I was running late.

Cheers!

(P.S. I'll make no promises, but I'll try and get today's log – and last week's log – up sometime soon.)

Another License Renewal

Well, just paid my license renewal for another year. Always a reminder that I've still not updated my license yet. The original plan (back in 2008) was to upgrade my license in the first year. Oh well, hopefully I'll have it done by the end of the third year. Fingers crossed!!

Hmm, actually I think I was first licensed in 2007… That makes more sense… So that'd be I'm into my fourth year…hmm.. confused.

Stacking Harness for Two 4el 2m WA5VJB Yagis

Today I started to get the parts for my phasing harness for stacking two of the 4el 2m WA5VJB Cheap Yagis. Basically this is planned as follows:

First, rather than attaching the cable directly to the elements this time (which are aluminium – as I'm building two more from aluminium that are planned to be identical) I plan to mechanically attach N sockets to the DE. Therefore today I grabbed the N connectors, and will soon also grab some aluminium strapping to bend into the shape required.

Then I need to organise the harness bit. Now the basic theory is, that when you connect two antennas in parallel they appear simply as two impedances in parallel. This means if both yagis are tuned correctly to an SWR of 1:1 and thereby appear as a 50 Ohm Zload (when connected with 50 Ohm cable), then if they were both simply connected we'd have the following:

Zin = (R1 x R2) / (R1 + R1)
Zin = (50 x 50) / (50 + 50)
Zin = 2500 / 100
Zin = 25 Ohm

So rather than having a SWR of 1:1 it'd be up near 2:1!!

Instead, we need R1 and R2 to appear as though they're 100 Ohm each. One way to do this is through the application of an impedance transformer. The impedance transformer I've gone with is to use a 1/4 wavelength (or odd multiple thereof – ie. 3/4, 5/4) of coax with a 75 Ohm cable. When this is then attached to a Zload of 50 Ohm, the Zin will appear as 100 Ohm. There by we'd end up with:

Zin = (R1 x R2) / (R1 + R1)
Zin = (100 x 100) / (100 + 100)
Zin = 10000 / 200
Zin = 50 Ohm

And there we'd have the SWR 1:1 we're all after. The basic setup would then be:

Two 4el 2m WA5VJB Yagis tuned to a SWR of 1:1, each connected to a length of an odd multiple of 1/4wl 75 Ohm cable (I've chosen RG-6). These two lengths can then connect into some kind of junction (still to determine) which can then be connected to with normal 50 Ohm cable – maybe some RG-213 or RG-8 – back to the radio.

(But keeping in mind that the lengths have to be the SAME odd multiple – so both 1/4wl or both 3/4wl – otherwise the phase will be out and potentially cancel or something.)

So the other item I grabbed today is some Belden 1189A (RG-6/U). That, combined with my N sockets puts me about half way along on the harness front. Still need some N connectors that will work with RG-6/U and also to decide what I'll do with the junction point РI'd love a nice simple N connector T Junction (but does such a thing exist??).

And one last thing I've learnt… Where I say 1/4 wavelength above, I mean ELECTRICAL wavelength – not mechanical. So you _don't_ simply do:

(300 / f) / 4
ie. for 144 MHz you don't do:
(300 / 144) / 4 =
2.083 / 4 =
0.52 meters

You HAVE to factor in the Velocity Factor (VF) of the cable chosen. Now ideally this should be measured, but the starting point is the manufacturers data. For Belden 1189A this has a VF listed as 0.82, so we work it out like this:

((300 / f) * VF) / 4
ie. for 144 MHz we should do:
((300 / 144) * 0.82) / 4 =
(2.083 * 0.82) / 4 =
1.708 / 4 =
0.427 meters

Once you've got that (which you can see is definitely different to the first calculation) you can start measuring it. I plan to do this with a dummy load and SWR meter; and the help of VK1OD's TLLC (http://www.vk1od.net/calc/tl/tllc.php). In theory if I hook up my planned piece of cable and all is correct I should get an SWR of 2.2:1.

Anyway, there you have it. My current understandings of a basic phasing harness and how far I've got. Could be wrong, and I've still got a bit to go, so I'll keep you posted. ūüėČ

(Ohh, and I've used 300 in my calcs above, but maybe I should use the 299.8 – as in the Speed of Light – but I reckon that'll do. Actually, I do wonder how accurate I can really aim for. Hopefully I can get it fairly accurate, I just wonder what process W5UN used when doing the harness for his massive EME array – linked to earlier. The errors there would definitely compound until it was unusable I'd think….)

What’s that Bird?

Well, of late I've noticed a bit of bird life around my location for the 2m FM Lunchtime Net. So I described it to the net and James (VK1DR) came back with the possibility of it being a Fairy Wren. So I finally googled it and it's indeed the exact bird I'm watching while talking to people on the net. He's a cool little character and adds some extra colour to the surrounds. Now I just need to take my camera with me one day (rather than my iPhone) and try and snap a pic.

Thanks James!