The BVO70 yagi series de DJ9BV
published in DUBUS Technik V 1998
After building some 2m yagis it is time to go ahead and build some powerful antennas for 70cm DX too. Here in central europe 70cm weak signal activity unfortunately isn't that high compared to 2m. So I need some real high performance and still portable antennas to go tophill. My first choice was the BVO series by DJ9BV, the latest publication of this well known author.
My actual choice was the BVO70-8.5wl (26ele). The boom lenght of this antenna matches right the size of aluminium tubes of 6m sold overhere. It is also a pretty broadbanded yagi you can use throughout the whole 70cm band.
These antennas are designed as 6l (4m boom), 8.5l (6m boom), 11l (7,5m boom) and 13l (9m boom) version. You can find datas for some of the other lenghts at G3SEK's Long Yagi Workshop. If you need some different measurements please e-mail me.
Boom material and support
For weight reason I choosed a square aluminium tube 20 x 20 x 1,5 mm. It is a good compromise in terms of stability and weight. Holding the antenna in place you can use a cord support as a riser or a 'trombone' style aluminium support (not with a cross yagi of course, in this case only a cord support ! ).
For different boom diameters you have to add the following amount to the free space lenght (only correct for insulated mounting through the boom):
Elements are solid aluminium rods with 5mm (3/16") diameter mounted inuslated through the boom. And here is there difficulties start. For 3/16" yagi element holders are obtainable from DownEastMicrowave. If these holders are suitable for 5mm I don't know, without modification (drilling) surely not. The option of purchasing these from USA isn't worth the money, just too expensive, especially if you are only building 1 or 2 antennas.
So I asked a friend (Manfred, DK9TG) for a solution and he was able to found a prototype which I received today. The functionality is the same as the holders from DEM do have. Two shoulder washers holding the element insulated through the boom. Stainless steel push-nut retaining rings slid along the element will secure it. The insulators are made from black polyamid.
You can find this kind of fixing the elements also in the ARRL Handbook and the ARRL Antenna book described in the chapter about yagis of Steve Polishman, K1FO .
If you are building just one single antenna it is maybe not necessary but as soon as there are more elements to cut I would recommend the cutting jig of G3SEK. Saves a lot of time and even EME arrays are no problem anymore.
Driven element is 8mm or 10mm(3/8") aluminium tube. But what kind of DE one should use ? First there is the classic folded dipole with a coax balun. It is a logical choice for a 50 Ohm antenna we have here. The designer do suggest this kind of DE.
The measurements are 317mm long/ 53mm high (centre to centre) from 8mm alu tube. The centre gap is 15mm.The balun is made out of RG-142 B/U double shielded PTFE coax cable.
So far the infos until now. I have to go out and do some drilling, sawing and filing. You'll see some further information as soon as possible here.