M105 hairpin

[paulsbox]

getting ready to setup a m105 with hairpin match. Splitting and isolating the driver element, does the length need to be changed or can I leave it stock?

 

[bob85]

If you start with a gamma matched setup & split the element to use a hairpin the element needs shortening to give it some capacitive reactance that the hairpin will cancel out.

 

[paulsbox]

will that cause all the elements to be changed or just the driver?

 

[bob85]

i have modded a friends 10mtr beam without altering the other elements & it worked ok,
it may not be optimal but what beam is optimal.

 

[Marconi]

Bob, a while back somebody asked me about doing a 4 element yagi with a hairpin match. I think I recall it did not look to perform as well. Such an antenna would require the elements be much shorter at resonance.

I think you have strongly suggested to us before that more metal in the air is generally better in antennas.

I have a real Hy-Gain 10 meter 5 element Long John. I got it thinking it might have enough metal to make it into 11 meters...WRONG. I wasn't counting on the beta match to require the antenna to have shorter elements.

Luckily, I had some tubing that fit on the ends and I was able to get the thing to resonance, but I found out I could not tune the antenna near the ground, so I was very limited to the height where I could tune insitu.

My homebrew 4 element yagi beat this antenna on all counts...even thought the antenna was nicely tuned...after I added an inline ferrite bead choke and got it a few feet above my garage so I could work on it with a little extra height. It was never enough...and the antenna was intended to be installed 2-3 wavelengths very well into the clear...so that might have made the difference.

 

[bob85]

Eddie
I have a Cushcraft 10-4cd skywalker that a local tried to put on 11mtrs, cut the boom down & ruined it then spliced the piece back on with a piece of scaffold pole & hose clamps,
the decent quality element tubes will come in handy.

The 10mtr beam i converted to hairpin for a friend was on 10mtrs & tuned up ok with a simple vswr meter, he wanted rid of the gamma, this was before the internet, the only book i had was an old ARRL antenna book.

 

[Marconi]

Eddie
I have a Cushcraft 10-4cd skywalker that a local tried to put on 11mtrs,

I have a Cushcraft - 15-CD and I think I have a old model for it. I built the antenna for 11 meters, but don't have a record in my file how it worked out.

The 10mtr beam i converted to hairpin for a friend was on 10mtrs & tuned up ok with a simple vswr meter, he wanted rid of the gamma, this was before the internet, the only book i had was an old ARRL antenna book.

I will try it using a hairpin match and let your buddy know what the model predicts for 10 meters...if my 11 meter works out.

 

[bob85]

it was many years ago Eddie before internet but id be interested in what happens when you swap from gamma to hairpin match without changing the other elements.

 

[Marconi]

Here is a large PDF file for an old Maco 105C model I had that was done using taper. I didn't intend to do this project that way but the old model I had that used a hairpin matching was old and was done back when I was modeling using taper. I did not realize this until I was finished doing both models.

Bob, if you think it makes a big difference I'll take the newer M105C model with the gamma and build a beta match on it like I was intending to do and see if it make a difference. I was able to get pretty good AG results on both models...so I don't believe there will be much difference.

The model in blue on the overlays is the model with the Gamma Match. I forgot to include this notation in the description.

The Hairpin model makes the model inductive and requires an 8" inch reduction in the overall driven element length...198 vs. 206 for the Gamma Fed antenna and I think the director elements should be proportionally smaller.

it was many years ago Eddie before internet but id be interested in what happens when you swap from gamma to hairpin match without changing the other elements.

I'll try and get this too.

 

[Captain Kilowatt]

I don't see why the parasitic elements would be shorter. There is nothing changed that affects their resonant frequency. I could be wrong but....... I built a six element 6m yagi with Beta match (hairpin) and the driver element was a couple inches shorter than the first director, which is odd when you first think about it, but the Beta match requires a shorter element. Not sure why any other length would have to be altered.