On 9/30/98 9:33 AM, Lowell, Mark at mlowell@noclant.navy.mil wrote:
>
>
>> >What would you suggest in it's place given:
>> >
>> >1. Less than 100 feet long.
>> >2. Covers 1.8 - 30 Mhz.
>> >3. No antenna tuner.
>> >4. Feed it with coax.
>> >5. Put it up in an afternoon with a couple of poles.
>> >6. $200 or less.
>> >
>> >Thanks,
>> >Barry
>>
>> I'd eliminate requirement #3. Then, I'd put up a 100' doublet, and feed
>> it with open wire or 450 ohm transmitting twin-lead. Run the open wire
>> right to the house, then use a balun before going inside. Use the
>> shortest possible run of solid dielectric RG-8 / RG-213 to get to the
>> tuner. Some transceiver auto-tuners can handle this antenna directly.
>>
>> 100 feet is a bit too short to cover 1.8 MHz. But, as long as it is high
>> enough, this antenna will do OK for the other bands.
>> Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@radio.org
>
>> I like AA4LR's solution, though I'd try it without the balun at the
>>house first, and I'd droop and/or dogleg as much wire as possible off the
>>ends and give that tuner a fighting chance on 160m!
>>73, DavidC AA1FA
>
>I can contribute a suggestion. Bill and Dave are right about a multiband
>doublet. They can and do work well.
>
>Make a doublet and feed it in the center with ladder line. Transition the
>ladder line to shielded-parallel lines where it enters the shack (2 pieces
>of rg-58 - connect per ARRL antenna book). Connect to an inexpensive tuner
>with internal balun similar to MFJ 949. Sorry, but you've got to have a
>tuner!
Let me add something to this. While using two runs of coax will work as
described, I would not recommend small coax such as RG-58, nor would I
recommend any coax using a foam or air dielectric.
Why? Well, sure, the losses in foam or air coax are lower, and even
RG-58A/U FOAM can easily handle maximum legal amatuer power WHEN PROPERLY
TERMINATED WITH ITS CHARACTERISTIC IMPEDANCE.
In this service, coax is asked to operate in an environment of complex
impedance. Which means that the magnitudes of resistance and reactance
change as you move along the coax. Now, the tuner takes care of this
complex impedance, and the entire antenna system operates at resonance
due to the conjugate match. However, the coax is the weak link.
Since the impedance is complex, substantial voltages may be present at
some points along the coax, even at low power levels. Since foam or air
dielectric coax has substantially lower breakdown voltage than solid
dielectric, you risk arcing in the coax -- which everyone will agree does
terrible things to your antenna efficiency.
Another note would be to keep the coax run as short as possible. Remember
that compared to the characteristic impedance, the coax may have to
operate with a substantial SWR (though the conjugate match from the tuner
guarantees a 1:1 SWR throughout the system). This greatly increases the
losses in the coax. (See, guys, all those battles and myths about antenna
SWR and efficiency have NOTHING to do with the antenna, and EVERYTHING to
do with the coax feedline -- if we go back to open wire, we wouldn't
worry about SWR)
LB Celik also warns that toroidial baluns may be a weak link in this
system, since they are being asked to transform a complex impedance. This
could cause core saturation at modest power levels.
>Now, here's the neat part. No ground radials will be required. Also, you
>can greatly extend the doublet's low frequency resonance by capacitively
>end-loading it, the most efficient way of loading an antenna.
This is a good tip, provided you have the doublet sufficiently high to
allow the 'tails'.
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@radio.org
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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