quarter wave inverted L questions
I have a small backyard and I currently have a G5RV Jr. (51 feet) at about 25 feet. It works better than I though it might on 40 10 meters. (I have a Kenwood TS 520S with a Kenwood AT 230 tuner so I am not able to access the WARC bands but I can tune an antenna) I would like to get on the 75 and 160 meter bands and given the size of my lot and the trees available I am leaning toward an inverted L with traps for 40 meters and 80 meters to help with the physical length. I can go up about 30 feet and out about 50 feet. After that I will have to dogleg the remainder which should be anywhere from a few feet to up to 20 feet depending on the traps I use. I base this on several designs I have looked at some of which are from hams in the UK dealing with small lots. I realize this is a compromise setup with respect to performance but it meets my needs.
Should I use a dozen radials ranging from 30 to 60 feet on the ground? or Should I use one, possibly two 120 foot raised counterpoises wrapped around the house and the detached garage? If I used counterpoises I might use traps in them to shorten them up a bit. They would have to wrap aound the house and or garage given my lot size.
I have experimented with building a coaxial trap for 40 meters using RG58 and 2 inch PVC and I can get a dip around 7.150 mhz. I can seal the coax with liquid tape or similar sealant but I am still a bit leary of the durability of this design given the weather extremes here in upstate New York. I have noted that Unadilla/Reyco type traps look quite durable and all things considered are reasonably priced. I think they might have a higher Q but less bandwidth some of which I could compensate for with my tuner. Any thoughts on the benefits of homemade versus commercial traps?
Thanks for any guidance with this project.
JT in Albany
Quarter Wave Inverted L's... Go for it!
You have enough room for full sized quarter-wave inverted L antennas. Lengths are roughly 234/F (F=frequency in MHz), or about 32-33 feet for 40M, and 66 feet for 75/80M.
Put up two, common feed point, but spaced slightly apart w/ spacers, quarter-wave inverted L's with a good ground radial system, and work DX.
Just be sure to concentrate on installing many (two or more dozen) radials, as long as you can make them, especially in the direction of, or underneath, the overhead horizontal radiator -- the more radials, the better.
Originally Posted by WA2BJE
Last edited by NL7W; 01-29-2010 at 11:49 AM.
73, Steve, NL7W
Not in but around Palmer, Alaska
Alaskans: Folks with Latitude!
Lots of shorter radials (mixture of 66 feet and 33 feet), fanned out from the base of the antenna on the ground.
A good choke at the base of the antenna too, to stop RF coming back down the outer braid of the co-ax.
Two radiating wires from the single feed-point - 1x 66 feet and 1x 33 feet, separated as much as you can for 80m and 40m.
Josh / M0JMO
As I understand the original post we are talking about a 160m/80m/40m antenna.
Coaxial traps are likely to be more lossy than discrete L/C traps, but I doubt you'd notice the difference in normal operation. I'd go with whichever looks more durable.
I'd definitely choose the ground radial option over two counterpoises wrapped around the house/garage.
I've been thinking about building an inverted L for 75m (66' total length; 25-30' vertical, 41-36' horizontal) and wonder what the performance would be like using a LDG Z-100 placed in a weatherproof box out at the base of the vertical portion. Of course using a short jumper to a balun since the z-100 only has coax connections. Something like that might get you away from having to use coils and be able to access bands other than just 75m.
K5BDH, you are proposing an unbalanced antenna fed by an unbalanced tuner. A balun in such a system serves no purpose. A balun is used at the interface of BALanced and UNbalanced parts in a system - hence its name.
73, Ron KR2D
your right that wouldnt make a whole lot of sense, you could just run a short piece of coax from the atu at the base of the inv L to the antenna and radials just like you would if not using an atu there. I'm wondering which bands one could expect to get using an atu there. I suppose you could do the same setup with a inv L cut for 160m, but would it get you 80 and 40m without the coil setup as mentioned?
In that application, a 1:1 current balun can serve the very useful function of preventing the outer surface of the coax braid becoming an unwitting extra radial and conducting current right back into the shack. Without the balun, if you don't have a low impedance ground the braid might be the prefered path for antenna return currents.
Originally Posted by KR2D
I ran an LDG-AT7000 in a waterproof box at the base of my 80m Inverted L for a while, until I started using my Linear.
I just got a short bit of co-ax with 1 PL259 to connect to the ATU, then connected the other end to my antenna using a connection block.
Worked pretty well from 80m to 6m, although on some of the higher bands I got some RF in the shack, which I can only assume came up the outer braid of the co-ax. A 1:1 Current Balun used as a choke would probably have resolved this though.
Josh / M0JMO
thanks for all the good info. Lots of ideas, now to just get them up and running. I just need to find a place with a good deal on a 30' push up pole. Another idea is to pickup a couple 10' sections of aluminum tv mast and use that as a vertical element with the remainder of the 66' as a wire attached at the top of the vertical.