[50mhz] 6m Yagi Antenna Conversion...

[Chris Boone]

(Is this the same Jason who is nephew to KB5BFA? & who's H/S graduation 
I attended 7 or so years ago????? ;)

Jason, for a quick antenna, especially when the band opens, a 1/2wave 
dipole (Hor or vert) or 1/4 ground place will work fine...your local 
range wont be great if you run vertical (since you would be cross 
polarized with the other SSB folks) but anything via skip won't care 
about it.....I picked up some mounts at EPO earlier this week....has 
SO239 connection and can mount TWO 3/8x24 threaded antennas 180 degress 
apart.....take one CB 102in whip, cut into two pieces (need an extra 
ewnd with stud and set screws for this) and mount the two whips as a 
horiz or vertical dipole...cut
both ends equally for min SWR at the desired freq range...

NOW to answer your question on the yagi, yes you can cut the elements 
down to freq and move the spacing to get it on the 6mtr band....An easy 
way to get the details right is model the dimensions on what you have 
right now...
if the antenna is cut for say 41.0 MHz, then determine what the constant 
is for the elements.....length x freq = constant...then divide the new 
freq into that (it looks like you MAY have done that already below...)
Always cut a little LONG to make sure you can tune to freq...kinda hard 
to make an element longer! You can ALWAYS cut shorter :)

As for the gamma match, #2 below is not concerning that...a gamma match 
is never that long!!! It should be about 10-15% of the element length..
on 6mtrs IIRC, a gamma match rod should be aboutr 12-18 inches in length 
tops! Look at the % length the gamma rad is now compared to the half of 
the driven element it is parallel to...use that same % wise length in 
adjusting the gamma for the 50MHz freq....BTW nothing says you cannot
leave the rod the length it is now and just move the tap...BUT it may 
induce some capacitance because of the added length running parallel to 
the driven element....and you may need to account for that in tuning..

Spacing on a yagi should be .2-.3 wavelength....different spacings 
between DE to Rf and DE to Dir will cause the pattern to change 
(especially side lobes and Front to Back ratio)...I dont have my docs in 
front of me but something about .23wl seems to stick out as optimum...
but a good optimizing program can tell you exactly what the spacing 
should be...(it will likely be different for the DE to Rf vs DE to Dir)

If you need anything else, give a holler...Im up in NW Hou next to Bear 
Creek off Hwy6.

Chris
WB5ITT
EL29et
listening 50.125


DUGAS, JASON A. (JSC-EP) (NASA) wrote:

> Hello all,
> 
> New to the list, new to VHF contesting, and new to 6m!!!  I have a 6m
> transverter on the way this week to use just in time for the CQWW contest
> this weekend... one problem NO ANTENNA!!
> 
> So I picked up a very old Yagi, built in 1969, but still in great condition.
> Its currently cut for the 40 MHz band, so I should be able to trim the
> elements down and change the spacing for a 3-element 6m antenna, right?
> 
> At any rate, the information I have been able to find so far (from various
> Yagi calculators found online such as that found at
> http://www.csgnetwork.com/antennae3ycalc.html) give the following figures:
> 
> Calculated Driven Side Element Length    4'6 19/32"
> Calculated Total Driven Length    9'1 6/32"
> Calculated Reflector Length    9'7 5/8"
> Calculated Element Spacing    3'7 7/32"
> Calculated Director Length    8'6 23/32"
> 
> Here are my questions:
> 1.  Can I do this?  Cut the elements, change the spacing.
> 2.  What does "driven side element" length refer to?  Is this the "matching
> dipole" (gamma match) part of the driven element?  
> 
> Are there any unexpected snags/pitfalls I should look out for?
> 
> Thanks,
> 
> Jason
>