Re: NEC-LIST:Loaded dipoles

Not so many years ago I run across a small helix antenna manufactered by a
well-known company. The antenna had an excellent SWR but didn't work very
well. The designer decided to make it black and moulded the whole thing in
carbon-loaded epoxy...

Regards, Peter

 At 11:58 2002-02-25 +0200, you wrote:
>Hi all.
>
>Many years ago I ran into a problem with a thin stainless steel antenna
for use
>at HF. The developers and teh intended purchasers only saw this beautiful
SWR.
>Problem was that at best the antenna was some 25% efficient. So depending on
>what you wanted to do, and how much spare power you had ...
>
>If on the other hand the antenna is intended for reception, you need to
look at
>so-called active antennas.
>
>Duncan Baker.
>
>Chuck Counselman wrote:
>
>> <tuli_at_bellatlantic.net> wrote:
>> >The antenna is multiband (40,80,120,160 MHz + 20 MHz, +/- 2MHz around
each).
>> >The idea is to feed the 160MHz monopole and get the match by proximity
>> >coupling for all other monopoles. I'm all set for all other bands
except the
>> >20 MHz.
>> >A matching network at the input will affect all the other bands (lower
>> >efficiency) which I am trying to avoid.
>> >This is why The only way (I see) is to wrap the 20 MHz dipole in a
resistive
>> >film. This way, only the 20 MHz monopole will be affected (see "The
>> >Open-Sleeve Antenna" by Roger A. Cox on CQ, August 19983 pp.13-19).
>> >The inductive loading would be by replacing the monopole with a coil ?
>>
>> Resistive loading of the antenna accomplishes nothing but to
>> dissipate power. If dissipation is your goal, you can get it more
>> easily with a resistive pad in the feedline between the antenna and
>> your transmitter and/or receiver. The only advantage of distributing
>> the resistance that I can imagine is to dissipate the heat into the
>> air more easily.
>>
>> To test my statements, use NEC. NEC can distribute the loading, as
>> well as lump it.
>>
>> You are correct that "a matching network at the input will affect all
>> the other bands." However, efficiency defined as power radiated
>> divided by power input is not affected, except trivially by resistive
>> loss in the matching-network components.
>>
>> The frequency range you give is 8:1. I have a very similar situation
>> myself, in my ham station. I operate all ham bands 80 through 10
>> meters, i.e., 3.5 MHz through 28 MHz. Here's what works for me. It
>> gives me a VSWR of 6 or 7:1 in the worst cases (7 and 14 MHz), and
>> under 1.7:1 on 3.5, 21, 25, and 28 MHz, excepting one pathological
>> special case of 17:1 at 10 MHz, which doesn't bother me because hams
>> aren't allowed to transmit more than 200 watts at that frequency,
>> whereas I can and do transmit 1.5 kW on all other bands. Since this
>> pathological case is at one-third of my highest frequency, it seems
>> equally irrelevant for you.
>>
>> My antenna is a horizontal wire 30 m long, fed at its center with a
>> parallel-wire transmission line having Zo = 600 ohms, and 12 m long.
>> Then there's a 4:1 balun transformer, then an unbalanced C-L-C "tee"
>> matching network to a 50-ohm coaxial line to my transmitter and
>> receiver. In other words, the matching network comprises a series C,
>> then a shunt L to ground, then a series C. This is a "high-pass"
>> filter configuration. It is fixed-tuned to give me a perfect match
>> to 50 ohms resistive at 3.7 MHz. I.e., I never adjust it.
>>
>> You could easily adapt my scheme to your situation by making it
>> unbalanced, since you say you have a monopole, not a dipole; and by
>> scaling it in frequency. My scheme is essentially lossless, as it
>> must be because I transmit 1500 watts.
>>
>> 73 de Chuck, W1HIS
>> --
>> The NEC-List mailing list <nec-list_at_gweep.ca>
>> http://www.gweep.ca/mailman/listinfo.cgi/nec-list
>
>--
>Tired of the Nigerian Scam/Spam being sent to you via YAHOO?
>Place a filter on YAHOO and sleep more soundly at night.
>***************************************************************
>Prof. Duncan C. Baker (Pr. Eng., FIEEE, FSAIEE, Sci. Nat.)
>Dept. of Electrical Electronic and Computer Eng.,
>University of Pretoria, 0002 Pretoria, SOUTH AFRICA.
>
>PHONE +27 12 420 2775 (OFFICE), +27 12 361 7480 (HOME)
>FAX +27 12 362 5000 (OFFICE), +27 12 348 5314 (HOME)
>E-MAIL: University: dbaker_at_postino.up.ac.za
> IEEE alias address: duncan.baker_at_ieee.org
> Private server address: apc_at_acenet.co.za
>LOCAL STANDARD TIME = UT+2 HOURS
>****************************************************************
>--
>The NEC-List mailing list <nec-list_at_gweep.ca>
>http://www.gweep.ca/mailman/listinfo.cgi/nec-list
>

-- 
The NEC-List mailing list <nec-list_at_gweep.ca>
http://www.gweep.ca/mailman/listinfo.cgi/nec-list