Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: New MFJ 1790 10 M Antenna  (Read 3014 times)
N1NQC
Member

Posts: 60




Ignore
« on: April 18, 2012, 06:04:05 AM »

Hey All,

Anyone know anything about or have hopefully tried the new MFJ 1790 10 M antenna. There is near ZERO on their website, which makes me cautious.

Anyone know if this antenna always needs to be  meter tuned  when assembling for "field" ops ? Or does it just go together in a "flat" match ?

Any ideas of the 1: 1.5 to 1: 1 to 1: 1.5 bandwidth ?

What is the gain (if any) in dbd, NOT dbi.

Any info appreciated,

de N1NQC
Logged
KI4SDY
Member

Posts: 1452




Ignore
« Reply #1 on: April 18, 2012, 07:35:43 AM »

It seems kind of expensive for what it is. I built an excellent performing 10 meter quarter wave ground plane antenna out of $20 worth of parts from Ace Hardware and Radio Shack.  Wink

It only has two ground radials, so I suspect the radiation pattern would follow the legs of those radials. If they added one or two more radials in a circular pattern and published some actual gain figures, they might have something. However, they are competing with many tried and true CB antenna designs at similar prices, so it may be a while before we get some user feedback on this offering.  Sad  
« Last Edit: April 20, 2012, 09:19:47 AM by KI4SDY » Logged
K3VAT
Member

Posts: 715




Ignore
« Reply #2 on: April 18, 2012, 08:16:40 AM »

Hey All, Anyone know anything about or have hopefully tried the new MFJ 1790 10 M antenna. There is near ZERO on their website, which makes me cautious. Anyone know if this antenna always needs to be  meter tuned  when assembling for "field" ops ? Or does it just go together in a "flat" match ?  Any ideas of the 1: 1.5 to 1: 1 to 1: 1.5 bandwidth ?  What is the gain (if any) in dbd, NOT dbi.  Any info appreciated, de N1NQC

Hi Kevin,

You're correct, a paucity of data about their 'new product'.  According to the MFJ homepage ad, the antenna has one 11' radiator and two 6' radials, so as such it isn't really a true ground plane as the radials are ~60% of the radiator (see ARRL Antenna Book, page 6-18).  Of course, one does not actually need four radials, two will suffice provided the antenna is elevated above ground high enough.  Perhaps they're using horizontal shorten counterpoise to get a better impedance match.  

Another 'caution' I noticed from the website, "MFJ-1790 has a super low angle of radiation to focus your signal directly where you want it and it reduces QRM/Noise."  What is 'a super low angle of radiation'? major lobe <5 degs??  How does one "focus your signal directly where you want it"??  No way on either of these.  This is not a multielement yagi erected at one-wavelength above ground.  Perhaps a decent low angle of radiation can be realized by mounting their antenna at the salt-water shoreline.  But IMHO this is hype.  Finally, how does it "reduce QRM/Noise"?   Isn't it well-known that horizontally polarized antenna have a better signal-to-noise ratio than vertical antenna, given all other things being equal?

Forget about any kind of gain with antennas such as these.  Perhaps one of our Forum gurus can model this for us and give us a bit more insight.  I'd take a wait-and-see attitude.

73, Rich, K3VAT


« Last Edit: April 18, 2012, 11:00:37 AM by K3VAT » Logged
W4OP
Member

Posts: 431


WWW

Ignore
« Reply #3 on: April 18, 2012, 10:46:36 AM »

My guess is that they are feeding this a bit like an off center fed dipole- but in this case in order to achieve a match closer to 50 Ohms.

Two or 4 radials is not enough to keep common mode off the feedline/mast.

The marketing hype about super low angle and low noise is just that- marketing stuff.
Build you own.
And what's wrong with dBi??

Dale W4OP
Logged
N1NQC
Member

Posts: 60




Ignore
« Reply #4 on: April 18, 2012, 12:40:39 PM »

 Thanks Guys.

I sort of smelled a problem here. It sort of reminds me of  the old Three Stooges joke : Do you know what this antenna is for ? It's for SALE !

There is nothing "wrong" with dbi, it's just that dbd  does not need the "correction factor" thrown in.

K
Logged
WB6BYU
Member

Posts: 13288




Ignore
« Reply #5 on: April 18, 2012, 12:53:18 PM »

Quote from: N1NQC
...

Any ideas of the 1: 1.5 to 1: 1 to 1: 1.5 bandwidth ?



Clearly there must be a problem with the antenna if it causes you to specify the
SWR backwards.  SWR is always a ratio of a number greater than or equal to one
number to one.  So 1.5 : 1 is a valid SWR, but 1 : 1.5 is not.  Getting it backwards
is often a sign that someone learned their antenna from dodgy webpages and is
likely to be misinformed in other areas as well.

Besides, there is no reason that the antenna necessarily reaches a 1.00 : 1 SWR
at resonance, so we'd just talk about the 1.5 : 1 SWR bandwidth.



Quote

What is the gain (if any) in dbd, NOT dbi.



The antenna is slightly shorter than a dipole.  Presuming the feedline is effectively
decoupled from the antenna, the gain will be slightly lower than that of a dipole
with the top at the same height.

The actual gain in dBi will depend on the vertical angle it is measured at, the height
above ground, ground conductivity, etc.  But basically it will work the same as a
vertical dipole, or as a quarter wave ground plane with sloping radials.  The current
distribution on all such antennas is pretty much the same, so the differences in
gain and patter will be minor.
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!