Reply
 
LinkBack Thread Tools Search this Thread Display Modes
  #1   Report Post  
Old March 13th 04, 08:53 PM
Bob Bob
 
Posts: n/a
Default 40m full wave NVIS loop - mulitband use

Hi All

I want to get some ideas on how to feed my horiz 40M loop (NVIS) for
harmonics of 7Mhz. It is sufficiently low to the ground that the feed Z
is close to 50r (which makes 14Mhz - 100r, 21Mhz - 150r amd 28MHz - 200r)

I realise that the radiation pattern may nit be desirable. Space is kind
of limited here and stealth is important

I want it to be "no tune/no ATU" and to avoid switching things in and
out. It is currently fed directly with coax and a 1:1 ferrite balun. I
cant use open wire feeder back to the TX (needing at least 10M of coax)

Cheers Bob VK2YQA

(bcnntp doesnt exist. If important use bcnoop)

  #2   Report Post  
Old March 13th 04, 09:25 PM
Reg Edwards
 
Posts: n/a
Default

I'm under the impression that NVIS propagation fails at the higher HF
frequencies - that is frequencies above the vertical Critical Frequency
which pass right through the E and F layers without reflection.

How often does the critical frequency exceed 14, 21 and 28 MHz for NVIS
operation to succeed? Which is better - day or night. Thanks.
----
Reg.


"Bob Bob" wrote in message
...
Hi All

I want to get some ideas on how to feed my horiz 40M loop (NVIS) for
harmonics of 7Mhz. It is sufficiently low to the ground that the feed Z
is close to 50r (which makes 14Mhz - 100r, 21Mhz - 150r amd 28MHz - 200r)

I realise that the radiation pattern may nit be desirable. Space is kind
of limited here and stealth is important

I want it to be "no tune/no ATU" and to avoid switching things in and
out. It is currently fed directly with coax and a 1:1 ferrite balun. I
cant use open wire feeder back to the TX (needing at least 10M of coax)

Cheers Bob VK2YQA

(bcnntp doesnt exist. If important use bcnoop)



  #3   Report Post  
Old March 13th 04, 09:32 PM
Dave
 
Posts: n/a
Default

lets see, no tune, no atu, no switching, only coax.... not too many
restrictions here. i would recommend one of those super automatic no power
no adjust antenna tuners that was sold briefly here in the states... until
someone x-rayed the sealed enclosure and found it was nothing but a 50 ohm
dummy load... worked relatively well with any length of wire over a wide
frequency range with no adjustment. similar arrangements have been used for
rhombics, adding a terminating resistor at the far end makes them much
broader banded. the same thing is done on Beverage receive antennas, a
resistor at the far end makes their pattern better. with this 'tuner' the
idea is to swamp whatever odd impedance the wire antenna presented with the
dummy load, at some frequencies you could get loss down to 3db or so, at
others it was much higher, but you never had to tune anything and it was
ready to go instantly on any frequency.

the more acceptable answer is to get a rig with a built in tuner or get a
small external tuner.

"Bob Bob" wrote in message
...
Hi All

I want to get some ideas on how to feed my horiz 40M loop (NVIS) for
harmonics of 7Mhz. It is sufficiently low to the ground that the feed Z
is close to 50r (which makes 14Mhz - 100r, 21Mhz - 150r amd 28MHz - 200r)

I realise that the radiation pattern may nit be desirable. Space is kind
of limited here and stealth is important

I want it to be "no tune/no ATU" and to avoid switching things in and
out. It is currently fed directly with coax and a 1:1 ferrite balun. I
cant use open wire feeder back to the TX (needing at least 10M of coax)

Cheers Bob VK2YQA

(bcnntp doesnt exist. If important use bcnoop)



  #4   Report Post  
Old March 13th 04, 11:00 PM
KC1DI
 
Posts: n/a
Default

On Sun, 14 Mar 2004 06:53:43 +1100, Bob Bob
wrote:

Hi All

I want to get some ideas on how to feed my horiz 40M loop (NVIS) for
harmonics of 7Mhz. It is sufficiently low to the ground that the feed Z
is close to 50r (which makes 14Mhz - 100r, 21Mhz - 150r amd 28MHz - 200r)

I realise that the radiation pattern may nit be desirable. Space is kind
of limited here and stealth is important

I want it to be "no tune/no ATU" and to avoid switching things in and
out. It is currently fed directly with coax and a 1:1 ferrite balun. I
cant use open wire feeder back to the TX (needing at least 10M of coax)

Cheers Bob VK2YQA

(bcnntp doesnt exist. If important use bcnoop)



I'm not too Sure exactly what you want to do with this antenna but as
you go up in Frequency the antenna will become less and less NVIS..
though it will make plenty of DX qsos. But I really do not see many
options for you except a tuner of some sort.. I use a little LDG Z100
Auto tuner here with my 160M loop which i'm feeding with a short run
of Coax. it work quite well on all bands 160 --- 10M and it takes on
seconds to tune.

73 Dave


  #5   Report Post  
Old March 13th 04, 11:03 PM
Cecil Moore
 
Posts: n/a
Default

Bob Bob wrote:
I want it to be "no tune/no ATU" and to avoid switching things in and
out.


Bob, if such existed, almost everybody would be using one. The best
I can offer is switched lengths of feedline, depending upon what
band you are on. For a 40m dipole, 1/2WL of ladder-line. For a 40m
dipole used on 20m, 3/4WL of ladder-line is about right.

I run a 130 ft. dipole on all HF bands with no tuner (per se) simply
by switching lengths of ladder-line. Take a look at my web page.
--
73, Cecil http://www.qsl.net/w5dxp



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----


  #6   Report Post  
Old March 15th 04, 01:37 AM
William
 
Posts: n/a
Default

Bob Bob wrote in message ...
Hi All

I want to get some ideas on how to feed my horiz 40M loop (NVIS) for
harmonics of 7Mhz. It is sufficiently low to the ground that the feed Z
is close to 50r (which makes 14Mhz - 100r, 21Mhz - 150r amd 28MHz - 200r)

I realise that the radiation pattern may nit be desirable. Space is kind
of limited here and stealth is important

I want it to be "no tune/no ATU" and to avoid switching things in and
out. It is currently fed directly with coax and a 1:1 ferrite balun. I
cant use open wire feeder back to the TX (needing at least 10M of coax)

Cheers Bob VK2YQA

(bcnntp doesnt exist. If important use bcnoop)


Bob, consider the terminated folded dipole.

http://www.cebik.com/wbfd.html

and

http://www.bwantennas.com/ama/amaindex.htm
  #7   Report Post  
Old March 15th 04, 08:52 AM
Bob Bob
 
Posts: n/a
Default

Hi Cecil

Yes I was very interested in your design. I saw it some months ago and
discarded it from the standpoint of being too long. One of the other big
differences too is that you are using feedline to offset the antenna
reactance whereas I am feeding a resonant antenna albeit at higher and
higher Z as the freq goes up. I only need to transform the Z.

I said "avoid" switching things in and out, but I can see that I am just
going to have to get a box and put something together. I was thinking a
broadband balun with maybe taps for diff frequencies or separate baluns.
I was hoping someone would present me with a "all in one" balun design
that would do it all! grin I was also thinking of using switched coax
based baluns (say 4:1's) but thought it a bit messy. The other "dream"
was somehow coming up with a magical lengths of some strange Z coax (say
RG62) and switching them in and out like your open wire design... (eg
1/4 wave Q sections 75r coax at 20m, 92r coax at 15m, 4:1 coax balun at 10m)

Some other notes

If I direct feed the antenna at 40M it only has a VSWR 2:1 BW of about
100kHz. I was already thinking of attaching a geared down
motor/fibreglass rod to roll up some of the antenna conductor (tinned
copper braid) for tuning. This would also serve to make up differences
on other freqs.

For those that queried NVIS not being so useful at a higher frequency.
This is more a case of making something work in the space available. The
loop is actually a very strange shape that roughly looks like a triangle
sloping down at maybe 30degrees. (The way the ground slopes) I havent
tried to model it but I would asssume that some radiation at harmonic
freqs would be desirably closer to the horizom.

Thank you all for your comments.

Cheers Bob VK2YQA


Cecil Moore wrote:
Bob Bob wrote:

I want it to be "no tune/no ATU" and to avoid switching things in and
out.



Bob, if such existed, almost everybody would be using one. The best
I can offer is switched lengths of feedline, depending upon what
band you are on. For a 40m dipole, 1/2WL of ladder-line. For a 40m
dipole used on 20m, 3/4WL of ladder-line is about right.

I run a 130 ft. dipole on all HF bands with no tuner (per se) simply
by switching lengths of ladder-line. Take a look at my web page.


Reply
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules

Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Question on antenna symantics Jimmy Antenna 28 January 27th 04 02:10 AM
Distance to Link Coupling in a Loop Antenna Al Antenna 6 October 28th 03 01:02 AM


All times are GMT +1. The time now is 10:18 PM.

Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 RadioBanter.
The comments are property of their posters.
 

About Us

"It's about Radio"

 

Copyright © 2017