[TowerTalk] more info on Re: 75 meter antenna

Sat Apr 9 11:14:49 EDT 2005

This NVIS configuration is included in the book "Near Vertical Incidence 
Skywave Communication" published by World Radio Books.  A drawing and text 
is on page 73 where it is called the Jamaica and is said to be used by the 
British Commonwealth Forces (military).  It is given a favorable efficiency 
but not too useful for the military due to requiring 6 poles and lots of 
room and wire.

My question is how useful is it for field day for sites that have dedicated 
40m phone and cw setups?  The book's drawing is in terms of wavelengths, 
however other articles in the book are confusing.  One article flat-out 
states that 40m is hopeless for NVIS.  Others show graphs of MUF (maximum 
usable frequency) with axis of 24 hour local time and MUF for several Solar 
Flex Index e.g. 75,100,125, 150, 200.  40m is shown to require the higher 
Solar Flex values else the signal will not be reflected back to earth. As I 
have read the numerous graphs, NVIS at 40 meters is a daytime operation only 
and is best during summer months and when the sunspot cycle is NOT at it's 
minimum as it is now.

Would it be worth the effort considering the Puget Sound location and the 
fact that many field day stations have fewer transmitter setups and will 
migrate to the higher frequency bands during the daytime?

k7puc

----- Original Message ----- 
From: "Al Williams" <alwilliams at olywa.net>
Sent: Thursday, April 07, 2005 9:02 AM

> EZNEC shows that this configuration is a barn burner for straight up
> propagation providing about 4 db gain over a single dipole and much
> attenuation at lower elevation angles.  However EZNEC shows that raising 
> the
> antennas from 15' to 20/25 adds about 1 db of gain.  NVIS articles and 1
> book that I have read say
> that the optimum height is about .2 lambda which agrees with my EZNEC
> modeling. Raising the antennas also makes the input impedance a bit more
> manageable.
> EZNEC doesn't seem to agree with the "...theoretical gain of about 7db" 
> as
> it reports about 10 dbi?
>
> It would seem that this configuration would be especially helpful in areas
> with high ham populations--not so good in Wyoming, etc.
>
> k7puc
>
>
> ----- Original Message ----- 
> From: "David Jordan" <wa3gin at erols.com>
> Sent: Thursday, April 07, 2005 7:14 AM
>>
>>
>> I run a NVIS on 40m  2 full wave dipoles fed in phase just 15ft above
>> the ground.   Within the first 50-75 miles the signals are extremely
>> intense. My station is an Internet remote so I actually operate about
>> 50miles from the xmit antennas.  Typically, I pin the "S" meter from 50
>> miles away.  The signals on 40m continue to be very strong up to about
>> 350-400 miles, then a dipole at 60ft begins to out perform the NVIS.  At
>> around 350-450 miles the two antennas are about the same most of the
>> time but when you get past 500 and well into 900-1000 the dipole at 60ft
>> is about 20db stronger than the NVIS. So, the NVIS does do the job for
>> closer in signals. The same would be true for 80m with the NVIS at about
>> 25-30ft above the ground.  The NVIS full wave elements are fed with
>> homebrew 600ohm opern wire (in phase).  The two elements are spaced a
>> half wave apart (on 40 that is about 70ft). The theoretical gain is
>> about 7db with a main lob angle of about 80 degree (straight up for all
>> intents and purposes) omin directional.
>>
>> Another nice benefit of the NVIS is that it doesn't hear the foreign
>> broadcast stations nearly as well as the dipole at 60ft.
>>
>> Your inverted V at 95ft would be great for DX but it's really too too
>> high for short distance communications.
>>
>> Having both a  NVIS and "V" at 95ft would be a lot of fun.
>>
>> 73,
>> dave
>> wa3gin
>>
>> Jim Miller wrote:
>>
>>>QST June 2002 Page 28 has an article for NVIS for 40 and 80.  Should be
>>>just
>>>what you are looking for.
>>>
>>>73, de Jim KG0KP
>>>
>>>----- Original Message ----- 
>>>From: "Joe Barnes" <n4jbk at bellsouth.net>
>>>To: <towertalk at contesting.com>
>>>Sent: Wednesday, April 06, 2005 8:33 PM
>>>Subject: [TowerTalk] 75 meter antenna
>>>
>>>
>>>
>>>
>>>>can anyone recomend an antenna for 75 meters that would be optimized for
>>>>zero to about 400 miles? I am currently using an inverted vee with the
>>>>apex at about 95 feet. I have 2  towers here , one is 100 feet the other
>>>>is 125 feet, they are 100 feet apart.
>>>>Thanks , Joe N4JBK
>>>>_______________________________________________
>>>>
>>>>See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless
>>>>
>>>>
>>>Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with
>>>any
>>>questions and ask for Sherman, W2FLA.
>>>
>>>
>>>>_______________________________________________
>>>>TowerTalk mailing list
>>>>TowerTalk at contesting.com
>>>>http://lists.contesting.com/mailman/listinfo/towertalk
>>>>
>>>>
>>>>
>>>
>>>_______________________________________________
>>>
>>>See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless
>>>Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with
>>>any questions and ask for Sherman, W2FLA.
>>>
>>>_______________________________________________
>>>TowerTalk mailing list
>>>TowerTalk at contesting.com
>>>http://lists.contesting.com/mailman/listinfo/towertalk
>>>
>>>
>>>
>> _______________________________________________
>>
>> See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless
>> Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with
>> any questions and ask for Sherman, W2FLA.
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>>
>
> _______________________________________________
>
> See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless 
> Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with 
> any questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
> 