The AM Forum

Looks like I'll be moving down to North Carolina sooner than expected.  :? I'm looking for suggestions on what antenna will blast a big signal up north to AM gangsta land. (160 75 40m)  I'm thinking about a wire beam. Coax fed dipole with a parasitic reflector? or dual driven elements phased?.. Delta loop beams?

Suggestions?....

sorry, don't have the money to erect a big log but will have plenty of land and trees!  :D  :D  :D

Hi Glenn,

Well, for 75M it depends upon your antenna supports. If 90' or higher and flat, then a 3el wire Yagi would be great.  If a single support and at least 170' high, then a 2el quad using a boom at the top with the quads hung off the boom ends using ropes to pull the quad loops out and away from the tower.

But, it appears you want to use high trees. What I would do is put up as many broadside FLAT! dipoles as possible, facing NE. Space them 1/4 wave apart. [They will look like a Yagi]  Run EACH equal length coaxial feedline into the shack. Now you have 4, 5 or 6! end connectors to do your magic in the shack. Feed them progressively 90 degrees with longer coax to each dipole heading NE. Or reverse this for SW. Or tie them all together for a local cloud burner.

If you can get these dipoles, even a pair, up at least 60' high, this system will give you the most versatility and clean gain pattern/f-b for that lower height - that you can get on 75M.

BTW, the coax feedline loss is NOT the total attenuation figure for all the feedlines, fortunately.... the power splits equally into each dipole, so the total db coaxial loss is really like having only one feedline.

For the other bands you mentioned: Put up a 3el wire Yagi on 40M at 60' high beaming NE, and a inverted L on 160M with a good radial system, plus a 160M dipole for the locals.

73,
T

Hi Tom,

Thanks for the info. Besides the ability to switch directions, what is the advantage of a driven system vs a parasitic array? Sure would be a lot easier to just put a reflector behind the dipole. (75 meters)

Thanks!

Hi Glenn,

Well, the main problem with parasitc coupling is that if the elements are at or closer than 1/4 wavelength above the ground...[maybe 60-80' on 75M is too close] then the ground coupling to the element gets too high and the Yagi elements themselves do not seem to couple well. I've tried many Yagis on 75M that look great on the computer when 50' high, but the pattern was real lazy and f-b poor.  But when moved up higher to 90'+ the pattern got sharper, as it should.

But, all driven element arrays with "forced" currents and phase and are obviously coupled well. I've seen f-b ratios of 20db with dipole driven arrays on 75M that were only 50' high - with GAIN!   It's EZ to get a good f-b, even with a loopstick and sense antenna. But GAIN without loss and ground coupling influences requires heavy conductors and good matches and reasonably high antennas in terms of wavelength.

So, in summary, if less than maybe 80'-90' on 75M, go with the driven array... if higher then a driven OR Yagi is FB.  That's why I suggested a Yagi on 40M, cuz most guys with decent trees can get it up at 60' [1/2 wavelength] w/o a problem.

73,
T

Got it Tom. I understand. Height.... I'll have to see how well my giant slingshot (the big shot) works down there on those big mawl trees caw mawn!

Your comments about height with parasitic yagi's got me to thinking about all the typical trap tribander installations, mounted 10 feet above the garage!  :badgrin:  :badgrin:

Yes, and traps are another whole problem area.  Some say that on 10M, the trap losses in some tribanders are so high that all you end up with is a rotory dipole... :-)  Or possibly some f-b... you can still have f-b with a good pattern but if the losses are high, then the gain is shot, so why bother?

BTW, on the dipole driven array.... you don't HAVE to bring the coaxes into the shack. You can use either a relay system under the antenna to change phasing or just hard-wire the whole system in one direction. Gary/INR has a clever relay system in the field remote controlled that works well.  He even hot switches it - That takes lots of BA's wid the 21XS on line.

T