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Re: Topband: Twin Lead Marconi Antenna

Subject: Re: Topband: Twin Lead Marconi Antenna
Date: Sat, 13 Jun 2009 22:10:08 -0400 (EDT)
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> Garry wrote: 
>> Years ago, Bill Orr wrote a book titled
Wire Antennas. On page 104 in 
>> this book, he described an
inverted-L type of antenna called a Twin Lead 
>> Marconi built
using TV win lead in the L section with a length of single 
conductor wire making up the last 17 feet of the L with a few radials. 
>> It was suppose to give a reasonable match to 50 ohm coax. Has
anyone on 
>> this forum ever built this antenna and what were
the results? 
Herb, KV4FZ wrote:
>> Gary,
For portable use this is a fine antenna IMHO. This is precisely 
>> the antenna I used for numerous DX-Peditions from South America
and rare 
>> Caribbean Islands in the 60's. I was compact to
pack along with the 
>> gear and was easy to get to work with a
nearby palm tree on the beach. 
>> The twin lead Marconi (mine
was 1/4 wave of 300 ohm twin lead over the 
>> entire length)
was an easy match for my Drake's T-4XC Pi network which 
>> fed
the antenna directly sloping out of the cottage window to the 
>> highest palm tree on the beach. For portable or vacation use
>> worked well and I was able to give many new
countries to those chasing 
>> DXCC on 160. 
>> I do recall however that the theory that the ground losses were
>> by a 4 to 1 base impedance multiplication factor
has been disputed. I 
>> always carried along some small 1/4
wave bell wire radials which 
>> terminated at the rig ground.
The twin lead Marconi may improve on the 
>> rigs ability to
match the antenna with a built in tuner but you can 
>> raise
the impedance by making a single wire the antenna longer like 180 
>> feet. This antenna can be generally brought to resonence by a
>> variable capacitor to remove the inductive reactance
it imposes. 

The problem with a 180 foot long inverted-L
antenna is that over half of the wire is horizontal so the majority of the
radiation is nearly vertical.  A series capacitor with a 150-160 foot
long inverted -L, preferably at least 70 foot up and the rest horizontal
will give a reasonable match and a lot less sky wave, with the high
current point 20-30 feet above ground.
>> The
twin lead is two close spaced for much bandwidth improvement and the 
>> ground loses are a factor of the radial system. But for certain

>> applications it is a good antenna to consider. 
> Herb, KV4FZ 
Again, use as
many radials as possible.  at least 30, each 65-70 feet long is a
reasonable minimum per ON4UN's many Low Band DXing books.

& GL,

George     K8GG
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