|
New to Ham Radio?
My Profile
Community
Articles
Forums
News
Reviews
Friends Remembered
Speak Out
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
|
|
1-10 of 12 messages
|
Page 1 of 2
Next
|
|
Fan Dipole and SWR measurement
|
Reply
|
|
by K6ELE on November 16, 2008
|
Mail this to a friend!
|
I just built the fan dipole from the hamuniverse web site that uses the SRI measurements. These include a minimum of 5.5in. in separation of antennas at feed point and 38in at end points. According to SRI the lowest freq. is cut 4% short and highest is cut 1% longer. I made it for 80 and 40 using 117 ft for 80 and 64ft for 40 for the phone band. According to SRI SWR should be 1.5:1 or better at resonance. I used k9YC's design for the choke balun, 6 turns of coax thru 5 Fair 31 torriods just below coax feed point. Antenna is 70ft hi and fed with RG 11 72Ohm coax per K9YC web site suggestion.
OK the problem, when I measure SWR with MFJ 259 its way over 25:1 for 80 and 40.Is the MFJ 259 with its 50 ohm input and RG 11 with 72 ohm a problem in getting a true measurement or do I have a design problem with the antenna?
K6ELE
|
| |
|
RE: Fan Dipole and SWR measurement
|
Reply
|
|
by VK1OD on November 16, 2008
|
Mail this to a friend!
|
The Hamuniverse design doesn't seem to specify the characteristic impedance of the feedline, perhaps they don't think it is relevant.
Substituting 75 ohm line for 50 ohm line in your application would not of itself account for the 25:1 VSWR you report.
Owen
|
| |
|
RE: Fan Dipole and SWR measurement
|
Reply
|
|
by K4SAV on November 16, 2008
|
Mail this to a friend!
|
Using your description and bare wire, that antenna should resonate at about 4.07 MHz and 7.8 MHz. The SWR on the 40 phone band is well over 25 to 1, but it is not that high on 80. You must have something else wrong besides having the 40 meter antenna too short. Another possibility is that AM broadcast stations are affecting the MFJ on 80 meters and you are getting a bad measurement. Try measuring it with a rig.
In a fan arrangement with the wires fairly close, the presence of the 80 meter antenna will raise the resonant frequency of the 40 meter antenna and lower its feedpoint impedance. The exact amount will vary depending on separation of the wires.
With the configuration you described, the impedance on 40 meters (after you correct the length) should be about 40 ohms. The SWR you measure at the end of the 75 ohm line will be a function of line length. Also the MFJ is calibrated for 50 ohm coax so you won't be measuring the real SWR. For example, with 150 ft of feedline you should measure a minimum SWR of about 2.0, and the point of minimum SWR will be about 120 KHz below the resonant frequency of the antenna. The real SWR relative to 75 ohms will be about 1.6 at a frequency about 120 KHz higher. These numbers will be different for different feedline lengths. (I know that's probably confusing - sorry.)
75 ohm coax would be a good match for separate dipoles but not for this fan arrangement, unless you run the 40 meter element at right angles to the 80 meter element. Then it will match a 75 ohm line.
Jerry, K4SAV
|
| |
|
RE: Fan Dipole and SWR measurement
|
Reply
|
|
by W7ETA on November 17, 2008
|
Mail this to a friend!
|
Did you check all of your solder connections with a multitester?
Do you get an open circuit reading when you check the PL259 in your shack?
What frequency do you get the lowest SWR reading?
73
Bob
|
| |
|
RE: Fan Dipole and SWR measurement
|
Reply
|
|
by W8JI on November 17, 2008
|
Mail this to a friend!
|
Most dipole antennas are close to 50 ohms, not 75 ohms. I'm not sure about a fan dipole because the elements all do interact a bit.
Let's just say it was 50 ohms and you had 75 ohm cable. The maximum impedance would be 113 ohms at the analyzer, which with a 50 ohm analyzer would read about 2: 1 SWR so it isvery possible the 75 ohm feedline could make your SWR read around 2.5 to 1.
Personally, I would not feed a dipole with 75 ohm cable. The systems we use are normalized to 50 ohms, and most dipoles at most heights are around 50 ohms plus or minus.
That might not be your problem, but it could be.
73 Tom
|
| |
|
RE: Fan Dipole and SWR measurement
|
Reply
|
|
by G3TXQ on November 17, 2008
|
Mail this to a friend!
|
I just modelled your dimensions in EZNEC using #14 gauge bare-copper wire at a height of 40ft wire over average ground.
The dimensions look way too short: 80m resonance is at 4.02 MHz and the 40m resonance is at 7.66 MHz. The predicted VSWR at 3.5 MHz is 26:1.
I suggest as a first pass you scale your dimensions, comparing the frequencies you want with the EZNEC resonant frequencies.
73,
Steve
|
| |
|
RE: Fan Dipole and SWR measurement
|
Reply
|
|
by WA3SKN on November 17, 2008
|
Mail this to a friend!
|
An SWR of 25:1 would indicate either an open or short somewhere! Break out the ohmmeter and start checking all your connections.
-Mike.
|
| |
|
RE: Fan Dipole and SWR measurement
|
Reply
|
|
by W5GNB on November 17, 2008
|
Mail this to a friend!
|
Check your MJF-259 meter against a dummy load. I found on mine that after repeted use of connecting and disconnecting cables to the SO239 connector and slightly twisting each time that I had broken the wire at the solder input of the connector inside the meter.
If that is the case, you will have to disassemble the meter enough to re-solder the connector and in the future do not twist the connectors as you plug in.
That one kept me puzzled for a short time till I realized what had happened.......
73's
Gary - W5GNB
|
| |
|
RE: Fan Dipole and SWR measurement
|
Reply
|
|
by N1LO on November 17, 2008
|
Mail this to a friend!
|
I agree about checking your connectors. Any mismatch this large usually turns out to be an open or short.
--...MARK_N1LO...--
|
| |
|
RE: Fan Dipole and SWR measurement
|
Reply
|
|
by W5DXP on November 17, 2008
|
Mail this to a friend!
|
Since you have an MFJ-259, try the antenna one dipole at a time. Chances are, you will be able to diagnose your problem.
--
73, Cecil, w5dxp.com
|
| |
|
Email Subscription
You are not subscribed to this topic.
Subscribe!
My Subscriptions
Subscriptions Help
Check our help page for help using
Forum, or send questions, comments, or suggestions to the
Forum Manager.
|
|
|
