Good evening all.
I'm currently in the process of building an LFA yagi.
http://hh.vk7.com.au:82/blog/index.php?c...id_post=39
I've done basic measurements with the antenna placed slightly above ground, pointing into the sky, using a half wave length of coax to avoid any impedance changes from what is at the feedpoint. Measurements seem good. Return loss is about 27dB, not bad.
I've now terminated the coax tail (RG-213) which runs down around the rotator (via preamp actually) to heliax. The feedpoint connection is as short as I could get it.
I've placed one ferrite core right close to the feedpoint (about 20mm).
https://www.mouser.com/ds/2/150/2643102002-346453.pdf
These ferrite cores have an impedance of approx 250 ohms at 144MHz. I seal them from moisture using sealing tape with electrical tape over the top for UV protection. I then route the feedline at a partial 90 degree bend straight down to under the boom, where there is a small loop and about 200mm further down the coax is three additional ferrite cores, so hopefully this will be enough to stop common mode current.
I've seen several designs of these yagis, and some have 1/4 "stubs" connected to the feedpoint at one end, then shorted to the boom. These allegedly create a DC short, and a "pseudo" BPF effect on the antenna. I'm doubtful of this effect, however, it is what it is. The DC short is of more interest to me, however, my second, and preferred option is to directly connect the centre of the far end of the loop driven element to the boom. As voltage is at a minimum here, we can do this. We get the DC short, and possible other advantages when it comes to static/noise.
I'm wondering what others have on their LFA yagis balun/stub etc wise, and interested in anyone who has built their own such as I.
Regards
Hayden VK7HH
Hi Hayden,
What you describe about 1/4 wave "stubs" connected to the feedpoint (at one end) and shorted to the boom (at the other) is generally known as a Pawsey Stub. Checkout Owen Duffy's dissertation on the subject here:
http://owenduffy.net/balun/G0KSC/index.htm
G'day Hayden,
You may need to add some more ferrites close to the feedpoint for them to be effective at stopping common mode currents. Try and get those additional 3 ferrites up with the other one at the feedpoint if you can.
It really does depend on what/where you read it, but some sources say above 1k at the required frequency, while I've also read recommendations of more than 5k !! But I do think that 250ohm at the feedpoint isn't enough, so the 200mm tail could well be radiating and throwing the pattern out.
Another method is a pawsey stub, but can be narrow banded.
I don't have the part numbers and specs on me at the moment, but I plan on placing about 6 or 7 ferrites at the feedpoint of the 2m OWL yagi, and 4 or 5 for the 70cm LFA.
Cheers
Jayson
P.S> the link to your blog site isn't working
Bugga, it looks like my work has blocked Haydens site.. I can access it from home though
Thanks for the replies guys.
Roger - OK on the Pawsey stub. I've seen many iterations of this, usually it involves the 1/4 stub connecting across the feedpoint and shorting out to the braid of the coax, not the boom. I assume the same effect will be had with shorting to the boom, as long as the 1/4 piece is electrically the right length.
Jayson - OK on the ferrites... I've already sealed with collar tape the other three further down
. I'll post a photo here if I can figure that out... I just didn't want the toroids drooping down within the elements, as you'll see from the photo...... here.
![[Image: 2psouvn.jpg]](http://i65.tinypic.com/2psouvn.jpg)
![[Image: 291gkuc.jpg]](http://i65.tinypic.com/291gkuc.jpg)
Last photo is upside down which is annoying, but I think you get the idea.
I do have more ferrite cores, I was saving some for my 6 metre yagi, but I'll probably need heaps more and there is better ways to do that.
So, if I cut the collar tape on those other four, and place them hard up against the first one, best practice?
Is it also better to do the 1/4 stub, limited bandwidth (could be a good thing?) or just screw the middle of the far end of the driven element loop to the boom?
G'day Hayden,
No worries on the upside down photos
I'm still learning the subject, so no expert by any means....
More cores at the feedpoint would be best, then route the coax straight back in line with the boom (straight over the middle of the reflector element) then loop it around and forward on the underside of the boom. That keeps it symmetrical - if that makes sense?
A Fully Symmetrical Coaxial Balun may be the best option, it uses two pieces of coax, one 1/4 wave length and the other 3/4 wave length in a configuration like this example half way down the page -
http://www.g0ksc.co.uk/creatingabalun.html but it introduces other problems like more joints for water to get into etc...
An example of a Pawsey Stub is on the same page.
I 'think' the screw in the middle of the far end of the driven element (at the zero volts point) is simply for static bleed off. I've used the same LFA with and without the screw there and didn't notice and difference in performance, although I don't have a super test gear setup.
Back on ferrites, these are the part numbers I've bought for the 6m LFA, 2m OWL and 70cm LFA
http://www.fair-rite.com/product/round-c...631101902/
http://www.fair-rite.com/product/round-c...643101902/
http://www.fair-rite.com/product/round-c...661102002/
I plan on using 4 on 70cm, 6 on 2m and a heap on 6m
Let us know how you go, building antennas is super interesting!
Hi Jayson.
That is how I have traditionally fed my yagis, from the DE over the back of the reflector then under the boom. On my old yagi I used three clamp on ferrite cores, which I think were not enough when I did it.
The distance between the DE and REF is 164mm which means I can theoretically fit 5 on there before the bend over the reflector. I could try that, or just see if they will go around the bend.
I have had limited success with the pawsey stub when I have tried to make it. I don't think the 1/4 stub is technically a "pawsey stub". I won't be mounting to the shield of the coax, only the boom, which I guess, is technically grounded if you have a good low impedance ground through the antenna mast clamps, mast, rotator, tower etc.
I have had static issues in the past with split dipoles on my yagis, thus one reason for going the LFA. I simply think the 1/4 stub does exactly the same thing, only that it has to be a 1/4 wave length for it to not affect the impedance at the feedpoint, adding losses too. I'm not so sure it's classed as a balun either.
The role of the ferrites will be to stop common mode current radiating down the coax line, forcing it to go back out the DE (if it is asymmetrical).
The ferrites you have chosen look good, they should work well.
Regards
Hayden
Hi Hayden,
I also share your concerns over Balun issues or:
Concerns with Balanced Feed and the possible radiation from the "hot"
feed cable.
The voltage at the "hot" spot depends on the antenna impedance so,
a 12.5 ohm split dipole feed will need a much lower feed voltage
than a 50 ohm LFA design (actually 1/2).
Since one always has "length" available between the feed point and
where one wants the, coax up the pole, to come to, a 1/4 wave piece
of, geometric mean of ( 12.5 to 50 ) impedance coax, eg. two pieces
of 50 ohm coax in parallel should do the trick.
Could this be the best solution?
Alan
d but a quick comment on the "Pawsey Stub Balun" Mine works like a treat and yes I copied the design from G0KSC, but what use is posting the details on the internet if you didn't want others to copy it.
I'm not taking credit for his modified Pawsey stub but it works for me, on 6m as well as 2m.
I hope you got yours sorted.
Cheers
VK4KX, Bernie