Single feed 2M and 70cm Yagi
Richard Clark G4DDP
I have been looking to replace the mish-mash of VHF/UHF aerials that I have on my mast at home. At the moment I have two 5 element 2M yagi’s one beaming NW and the other SE, 4M &6M horizontal dipoles beaming NW – SE and a 6M, 2M & 70cm collinear on the top, a bit of mess due to not putting a rotator on the mast ! Due to the derogatory comments received from the XYL saying it looked a bit of a mess I decided to take the opportunity of simplifying the installation and also make it more useful by installing a rotator as well.
As I was going to keep the collinear I decided that I would look to try and install two multiband yagi’s with a single feed rather than the mix I have at the moment. Logically this means splitting into two systems, one covering 4M & 6M and one covering 2M & 70cm. 4M & 6M is no problem as Justin G0KSC has several designs for this on his excellent website so I will decide between a 3+3 or 4+4 element for those bands. If you look at link http://www.g0ksc.co.uk/ there is a wealth of information.
2M and 70cm was more of a problem I thought until I looked around and found the website of DK7ZB who is a fount of knowledge on various aerial designs. If you navigate to his website and look on link http://dk7zb.darc.de/Duoband/duoband_principles.htm you will find some very interesting information on single feed 2M & 70cm yagi’s.
I decided to make the 5 ele 2M / 8 ele 70cm version using 8mm element tubing as I had “ liberated “ sufficient quantities when my old work place had closed down, I used about 7M of tubing in total to build the aerial. The boom was made of a 1.5M length of 1 inch square aluminium tubing. I used standard element holders that I rescued from old aerials but Justin G0KSC has nicer element mounts for sale.
The 2M dipole feed was mounted in a 80mm square plastic box and fed via a 5 turn choke of RG174 coax onto a N socket.
All the dimensions for the aerial are on DK7ZB’s site so I haven’t replicated them here. Once I built the aerial I then mounted it on a portable mast at about 10 feet and tested via a 2M and 70cm aerial analyser. Whilst the matching at 2M was almost perfect as shown in the article I found that 70cm beam resonated at 439 Mhz and after playing with the elements I found that by increasing the length of 70cm director 4 I could get the antenna to resonate at 432.2 MHz and 144.3 MHz with 50 ohms impedance. A check showed that I could tune across the whole of 2M without SWR rising and from about 431 MHz to 433 MHz with SWR remaining constant at 1.2:1.
One strange thing I observed is that the 70cm element lengths vary in length and do not slowly decrease as you go from director 1 to 7 but it seems to work. After the alignment I then put the portable mast up to about 25 feet and connected my FT817ND and carried out a few tests with 2M and 70cm beacons to ensure that the aerial was working as it should, the front to back ratio seemed fine with gain and directivity fine given that this was just a crude test.
Opposite is a picture of the completed antenna, now all I have to do is make the 4M/6M aerial and remove the old aerials and mount the new aerials.
The length of the aerial is 1.5M and maximum width of 1.04M.
4M 1/2 wave mobile aerial
If you look through the DK7ZB site you will also see an entry 50 & 70MHz verticals which describes the construction of 1/2 wave aerials for these bands by DK7ZB for 50MHz and OZ2M for 70 MHz. As I was also looking for a more effective vertical aerial for mobile on 4M and this looked very simple to build, this is what I did.
I built the aerial using an old Hustler mast of 1.43M together with a 70cm steel whip making a total length of 2.13M. As a halfwave presents high impedance at the feed point I used a PI network to match the aerial. C1 is 20 to 110pf variable, C2 is 2 to 15pf variable and L is 5T of 1cm diameter all mounted on pcb in a 80mm*30mm*30mm diecast box as shown in the article courtesy of DK7ZB & OZ2M. http://dk7zb.darc.de/start1.htm
The resulting aerial worked exactly as described and resonates at 70.4 MHz and covers the entire 4M band. The slight disadvantage is that the complete aerial is 2.2M long and adding that to a 1.65M car roof makes for interesting drive at 70mph plus avoiding low trees but comparisons with a standard 1/4 wave for 4M show that on the IC7100 comparing the two using the G0WYG station on 70.4125 MHz gave a +ve difference of 1.5 S points for the 1/2 wave on IC7100 S meter, crude reading but definitely much better.
Aerial Mounted !! Matching unit
73
Richard G4DDP/G8BXC
