40m vertical with manual tuner at base
Just wanted to get a quick opinion on a future antenna install for 40m. I'll have a vertical pole 30'-34' tall, with two radials at 90 degrees from each other at a max length of 35', unless bending one is optional, then I can go well over that.. Unfortunately, that's all I'm able to do though. So with that there, I figured, instead of using a coil at the base, I have an mfj-901b manual tuner that I was going to put in a water/weather-proof container at the base of the antenna, tune it for the center freq of 40m, and be on my way. How does this sound?
That should work, any chance of making the vertical ground mounted? Then you might be able to get more radials out on or just below the ground, they don't have to be that long either because they won't be resonant, just put lots of them down.
Marconi Antenna --- Quarter-Wavelegth Vertical
A 33 foot vertical with 33 foot radials is resonant at: 7.091 MHz, CW/Digital portion of 40 meter band (USA).
~ 37 ohms (Z) feed point impedance.
At 80 meters, the height becomes more substantial
At 160 meters, a 1/4-wave vertical is comparable to the commercial AM broadcast stations --
that use tower section for the antenna's driven element.
Last edited by W9GB; 08-05-2012 at 05:31 PM.
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I thought I was ground mounting it, unless I'm just getting confused? hah. Well, here's a picture of my house and what I have to work with. If the length doesn't matter to much, I can burry (an inch or two) a few out into the yard?
These are other neighbor's yards on either side and to the rear?
Originally Posted by KF5JAK
You are overestimating the length of the radiator (already stated).
Yes, you can put lots of short radials, or coated rabbit/chicken wire down. Keep them straight, avoid 90 degree turns. Just add another short radial from the base. (vs a longer bent single radial)
Yes, you can put a layer of dirt over it, but don't bury it deeply. If you use landscape staples, you can tack the wire down into short grass, and within a few weeks or a season it will "disappear".
If you are only using it on 40m, skip the tuner and just trim the radiator for lowest SWR. You should be able to get it to 1.5:1 or better. The base resistance will probably be higher, and you may get a very close match to coax (due to the added ground loss).
ps 20 gauge bare or enameled wire can be bought in large spools rather inexpensively. It is strong, hard to see, and makes good radial wire. Just use a lot of radials. Random lengths are ok. Dog fence wire is pretty cheap and will work, but the 20 gauge on spools (see Amazon or EBay) is cheaper.
Last edited by K1DNR; 08-05-2012 at 05:50 PM.
Id put a hygain av-640 on the center of your roof, guyed to the edges of your roof at 2 elevations. that would work fantastic. It already has a counterpoise, no radials needed. Will work far better then your current proposal. 2nd choice would be a Hustler 4/5/6BTV mounted the same, but with radials going over the roof.
Ok, so, that did answer my question about the radials and the tuner, thanks K1DNR, and yes, those are others yards around my house (the red/brown roof). However, if it would even be worth it, I'd like to tune it to 80m every now and then. KF6ABU, that would be great! Trust me, if I didn't live with other "non-hams," there would be antennas everywhere. But I have to keep it neat to avoid whining from the others on the lease.. I had thought of getting the 5BTV, but I don't have the money right now to buy anything. If I did, I would already of ordered the 5BTV.
I made an assumption you had to operate under stealth conditions. A QW vertical for 40m might be tough to tune with most auto-tuners on 80m. But it really depends on what you end up with installed at your location.
Originally Posted by KF5JAK
You could add some thin "invisible" wires symmetrically on the top - and top load the vertical. This would bring the series resonance (and the SWR dip) of the vertical to a point that would be easier to tune on 80m and also less lossy than tuning the 40m vertical alone. The wires would be connected at the top, and have insulators at the ends. Use strong mono-filament fishing line (60lb) to tie them off, and to guy the mast. You want them to be symmetrical and opposite (e.g. two wires directly opposite each other and exactly the same length) so the current cancels. (or add wire to the top, run to the house or a tree and create an inverted-L for 80, put a trap at the top to have a true dual band antenna, or clip the "extension" on when you want 80m, and remove or clip or short the extension to the base of the vertical when not needed)
Take a look at the ON4UN Low Band DXing book for a lot of ideas and information on the performance of verticals at 40m and below. Lots of info on short verticals and a chapter on low band antennas in small spaces.
Last edited by K1DNR; 08-05-2012 at 08:24 PM.
Yes, the 32-34 foot vertical should be tunable to 40 meters, and it will probably work easily on 15, too. 31 feet would be a good length for use with an autotuner at the base for 'all band' operation.
A lot of short radials is better than one or two long ones. On 40 meters, 25' is long enough over normal ground. On my 43 foot vertical, I have about 20 radials, lawn-stapled to the ground. I think 6 are 43 feet long, 6 are 25 feet, and the rest are 12 feet. It works very well on 40,30,20 and 17, poorly on 160,80, 15, 12, and 10 , but it does work there. Radials are the key to success.
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