[TowerTalk] Liberal Arts Question on Dipole Antennas
ersmar at comcast.net
ersmar at comcast.net
Thu Jun 1 11:06:01 EDT 2006
I have a couple of suggestions for you to try. I'm assuming you are using an antenna analyzer to check SWR. If you are using your rig's SWR meter, that will change some fof the tests I suggest, but I'll let you know which ones. You will also have to climb the tower a few times to conduct these tests.
1. Climb the tower and drop the 80M dipole. Then measure the SWR on the 40M dipole. If it's different from before (better or worse), then you know you have some kind of interaction between the two wire antennas.
2. If you see no difference in SWR on the 40M dipole, climb the tower again, this time with a 1W 50 Ohm resistor connected to two small alligator clips. Remove the coax from the Budwig connector and connect the 50 Ohm dummy load across the coax. Go down the tower and measure the SWR with your analyzer across the entire HF spectrum. You should see 50 Ohms plus a small amount of reactance across most of the HF spectrum. Things might get flakey up at ten meters. YOU CANNOT DO THIS TEST WITH YOUR RIG AND A 1W RESISTOR. If you see 50 Ohms with the above test, your coax is ok; if not, it's not.
3. You didn't say how close to the bracket you have the Budwig connector. If it's tight up against the metallic bracket, you might try lowering it a bit on a rope about two feet long. This will put a bit more distance between the antenna feedpoint and the tower. Easy to do and doesn't co$t much.
4. If there is no interaction with the 80M wire and your coax tests ok, then I believe your antenna is OK. You might want to put this antenna on the opposite side of the tower, where the 80M dipole was, and string out the wires flat, like the 80M set. This will raise the effective height of your (now horizontal) dipole to 45 feet, a bit better than the average height of your Vee - about 32.5 feet (average of 45 and 20 feet.)
With the 40M VEE at an average height of 32.5 feet, you have a high-angle radiating antenna - an NVIS (Near Vertical Incidence Skywave) antenna. Your signal will shoot almost straight up into the ionosphere and splash down to earth within a few hundred miles around you. Your signals in that area will be loud and clear. But DX won't even know you're around.
Bringing the ends of the wire up to more horizontal, and at a higher elevation above earth, you will bring the main lobe of your dipole pattern down a bit, shooting more signal off toward the horizon than directly skyward.
Gene Smar AD3F
-------------- Original message ----------------------
From: Lee Buller <k0wa at swbell.net>
> Ladies and Gents....
> Recently, I had to replace my 40 meter dipole (more of an Inverted-Vee)
> antenna because the coax center lead had broken off the. The dipole worked OK,
> but I was never really sure that I liked it. What is there not to like about a
> dipole antenna? (I sure I get some wise cracks here). The dipole did not seem
> to have a resonate point. It was below 1.8:1 on CW and was about 3:1 on SSB.
> So, the antenna was long.
> The new antenna, using a Budwig connector this time, shows the same
> characteristics. I cut the the antenna at 66 feet and I made sure that both
> sides of the antenna were the same length....33 feet. The antenna is mounted at
> 45 feet using a bracked that runs through the tower. The antenna in question is
> on the east side of the tower while an 80 meter antenna is on the west sime of
> the tower mounted on the same bracket. The feed points are about three feet
> apart....on seperate coax feeds.
> The end of the Inverted-Vee on 40 comes down to about 20 feet off the
> ground...while the 80 meter antenna ends about about 12 feet off the ground.
> The 80 meter antenna is in a straight line, while the 40 meter antenna has a
> slight VEE to to the tie points. So it has VEE to the the east....and a Vee
> down. This is hard to explain in words. I hope you experts get this.
> The new antenna was failry good on SSB but not great ... about 1.8. I
> attached two test leads to each end and the antenna show that it was at 1.8 on
> CW...around 7.025. It seems that the test leeds were too long and now it is
> resonate below 7.0.
> But, there seems to be a crazy problem that I cannot get the anntenna any
> lower than 1.8 to one....thinking that there should be a length in there that
> should be "perfect." I cannot find the "perfect point" which I would like to
> have around 7050.
> Reading the Antenna Handbook...this antenna is working as stated although I
> did not anticpate the length of an inverted Vee to go longer. I've read the
> articles, but it seems there is something wrong with the antenna.
> Is there an interaction between the tower...the 80 meter antenna...the Vee to
> the east?
> Is the coax bad? IT has been up there for quite some time and it was used at
> another location before I put it up here.
> Comments, flames, postulations, editorials, and pontifications are all
> welcome. They might show on my web pate later on....so...you have been warned.
> Thanks for the discourse on a simple dipole
> Lee Buller - K0WA
> In our day and age it seems that Common Sense is in short supply. If you don't
> have any Common Sense - get some and use it. If you can't find any common
> sense, ask for help from somebody who has some common sense.
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