antenna delta match impedance 2

[LPK49]

Anyone know where to find the equations for dimensions of a delta-match for a desired impedance? I find dimensions for 600ohm delta match everywhere. A 300ohm should tap a little closer to the middle of a half-wave dipole. I can't seem to find equations for this anywhere online. I know that using the 600ohm delta with a 300ohm line is not to bad, but would prefer to use the correct dimensions.

 

[MagicSmoker]

There are equations, but delta matching is usually done by "feel", using an SWR meter and a transmitter at low power. Here's one link that has the equations that get you reasonable close to the right dimensions:

http://www.ycars.org/EFRA/Module C/AntMatch.htm

 

[LPK49]

Thanks for taking time to answer. (I'm new to this site.) ...but, I've seen this and it is is for 600ohm like all the others. I'm making a single channel VHF receiving antennas not transmitting, so I can't feel my way with with a transmitter I do not have. I know hams like 600ohm and 50 ohms, but everything else is 300 or 75ohm. It seems that only hams post stuff on the topic of antennas. They write all the books too. I do appreciate their effort.

 

[VEBill]

You're correct about "...it is is for 600ohm like all the others." I have a pretty good collection of antenna handbooks and, as you stated, they all provide the same information.

For VHF receiving antennas, matching is typically extremely (!) non-critical. For example, I designed (using freeware Yagi design software) and built a batch of TV channel 8 Yagi antennas (to address a local problem). The 'driven' element was folded, and thus nominally about 300 ohms. But the Z would have been lowered by the other elements to about 200 ohms. I just ignored that little detail and used a little TV balun to connect the driven element to the 75 ohm coax. The antennas worked perfectly and provided superb reception.

So just build your prototype and tweak it for best results.

PS: 300 ohm twinlead is kinda sorta almost arguably obsolete. Most would use coaxial cable.

 

[LPK49]

Thanks! I really expected that it would work, but wondered why no one seems to have done the analysis in public. I too am making a channel 8 TV antenna. I also know that 300ohn twin lead is virtually obsolete but a balun to 75ohm coax is not, which is why the 300ohm folded dipole is so sweet. I like the physical arrangement of a delta-fed Yagi for its simplicity and lack of insulators. There is a formula for impedence (in Wicki) of a dipole as a fucton of feedpoint distance from the end, but it seems to be for a split element and does not give 600ohms for the dimensions of the commonly published delta. I think one could derive equations if you had some way to calculate the division of current at the delta contact points, but might also have to include the antenna effects in the legs of the delta. Maybe that's why its not out there. Thought it would be an interesting derivation for some smart student. If you have stayed with me this long, just one more question. Would it be reasonable to connect two delta-Yagis in parallel feed to get a 300ohm balanced feed point. I suspect you could just connect the feed points of two deltas. That puts the Yagis about 0.3 wavelenths apart, and I wonder about interacions. I guess I'll just tinker as I usually do. Larry

 

[VEBill]

Don't bother. Just use Yagi design software to design the element spacing, etc. Use a folded dipole and feed it into the 300-to-75 ohm balun.

I used electrical conduit for the boom and stiff ceiling tile hanging wire for the elements. The only difficult bit was the plastic insulation for the dipole.