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Author Topic: 24 inch 20m antenna ?  (Read 9094 times)
KD8PGB
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Posts: 138




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« on: January 07, 2012, 11:41:48 AM »


 Would it be possible to build a functional 24 inch 20m antenna for mobile that would resonate? I know it sounds ludicrous, I have an ATAS 120 but it is a pain to put on and take off every time I park in the garage, I would like to permanently mount a 20m antenna on my truck and only have 24 inches from the top of the bed for clearance pulling in and out of the garage.

 
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AA4PB
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« Reply #1 on: January 07, 2012, 12:11:25 PM »

Yes, you can make it resonant. You can also make it have a 50 Ohm feed impedance. What you can't do is make it radiate much signal. Most of the applied power will be converted to heat in the loading and matching coils.

Will it make contacts? Yes. I've made contacts with an 8-foot Hamstick on 75M. For 100W in you radiate about 2 watts. You are on par with a 2W transmitter into a dipole (at the same low height).
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WB6BYU
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« Reply #2 on: January 07, 2012, 01:50:21 PM »

Yes, you can.  A standard resonator on a short enough whip could be resonant, and
might even be 50 ohms due to the high losses.

You can even make it somewhat efficient if you do a fair bit of work with copper
pipe, either for top loading (like a DDRR antenna) or making a tuned loop antenna.

For example, a DDRR might be 4' square and 2' high.  The dimensions could be
adjusted to fit your truck, and you can use either an end capacitor or a loading
coil near the end for fine tuning.  If you provide ground loop around the bottom
under the antenna you may be able to keep losses less than 10dB using thick
enough copper pipe.

A tuned loop (or the misnomer "magnetic loop") also made of copper tubing would
require a high voltage tuning capacitor, ideally one that can be adjusted remotely
from inside the truck if you want to operate over more than about a 10kHz window.


For a conventional mobile whip you'd probably be better off using a flexible whip
on the top section and letting that bend over as you go in and out of the garage.
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K0BG
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« Reply #3 on: January 07, 2012, 03:04:45 PM »

You have the answer you were seeking, at least partially, but here is something to thing about.

A quarter wave vertical, assuming it is over perfect ground (it never will be in a vehicle), will have an input impedance of ≈36 ohms. The majority will be radiation resistance (Rr). However, since the Rr is a square function, if we halve the length (1/8 wave) the input impedance is ≈6 ohms. In order to resonate the antenna, we add a lumped inductance to cancel the capacitive reactance a shorter than 1/4 wave antenna will have. That coil will have resistive losses. If the coil Q is low enough, the resistive losses, along with the inherent ground losses a vehicle has, could actually bring the input impedance back up to ≈50Ω. A low SWR in other words. But in this case, the efficiency is a lot lower due to the resistive losses. We can adjust the position of the coil within the antenna, add capacitive top loading, or build a higher Q coil, but we're still shy of a full quarter wave antenna.

It should be evident then that all HF mobile antennas are a compromise. The only real question which needs to be asked is, how much of a compromise are you willing to put up with?
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AC4RD
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« Reply #4 on: January 08, 2012, 05:57:34 AM »

As everybody else has said, you CAN build an antenna that size that would resonate ... and you would find it not very good, probably.  There's also a commercial HF whip, hamstick-style but only 3 feet long.  I played with one of those and it worked better than I expected, but that was still pretty poorly compared to some other antennas I've been playing with.   Do you have to have the antenna mounted so high?  

I know "middle of the roof" is the best place in general, but that isn't an option for me.  I've also got a garage problem and only a couple of feet above the top of my car.   I'm using a bumper level mount (actually the "towing eye" on my car.)   For my own situation, using Hustler resonators on a DX Engineering mast, with the resonator clear above the top of the car, has been the best I've found so far. 
« Last Edit: January 08, 2012, 06:16:50 AM by AC4RD » Logged
KD8PGB
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Posts: 138




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« Reply #5 on: January 09, 2012, 02:23:42 AM »


 Unfortunately the answers were as I expected, severe losses resulting in a QRP signal...

I think for me the better answer would be to put a linear motor on a hinge to raise and lower the ATAS 120 as I move in and out of the garage, then I can have access to up to 40m, albeit with severe losses as well as the ATAS is not a very good antenna but I think it is probably better than the alternatives.

Thanks for your input guys!



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AC4RD
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« Reply #6 on: January 09, 2012, 04:11:59 AM »

I think for me the better answer would be to put a linear motor on a hinge to raise and lower the ATAS 120 as I move in and out of the garage,

When you get it built, please put some photos online, and tell us about it here on the Mobile forum.  I've toyed with the idea of a motorized foldover between my resonator and the whip on top, but I've never been able to come up with anything workable.  I'll be interested to see how you handle it!  GL!
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KH6AQ
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Posts: 7718




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« Reply #7 on: January 09, 2012, 05:55:58 AM »

The ATAS-120 is not a bad antenna when mounted high on a vehicle. I have one mounted at the roof line of my car and on 40 meters the radiation efficiency is 10%. On 20 meters and above the radiation efficiency is quite a bit higher and I can hold my own with home stations using verticals.

For best signal mount the ATAS at the truck roof line rather than on top of the bed wall.

I too have the task of removing the ATAS-120 when entering and leaving the garage and forgotten a couple of times. It takes only 15 seconds to unscrew the ATAS from the UHF mount.
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KI4SDY
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Posts: 1454




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« Reply #8 on: January 11, 2012, 05:08:52 PM »

I think that the closest you are going to get for what you need is the new MFJ 2320T short 20 meter hamstick antenna for $19.95. It is 36 inches tall extended and collapses to 25 inches. You can put a spring on the antenna base if the one inch is going to cause a problem. They have 40 meter and other band models as well. Check the ham.net reviews on this antenna!  Grin  

Cheap, fast, easy and it will work!  Wink
« Last Edit: January 12, 2012, 08:40:48 AM by KI4SDY » Logged
KH6AQ
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Posts: 7718




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« Reply #9 on: January 12, 2012, 03:52:34 PM »

I have an MFJ-2320 and on a mag mount on the trunk with a 4" GND wire to a screw it works well. 5 watts CW and many contacts.
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