| Author |
Message |
    
Charliebrown
Advanced Member
Username: Charliebrown
Post Number: 622
Registered: 10-2002
| | Posted on Friday, May 18, 2018 - 12:00 pm: | 
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I have a three element 2 meter MFJ beam antenna. Works great. but, I have a dual band 2 meter , 70 cm radio. I have great SWR on two meter but, even better SWR on 70 cm SWR on two meter average is 1.2 to 1.4 on 70 cm it is near flat 1.1 or less Now here is my question. With the SWR that low on 70 cm can I use it on 70 cm ?? I know it is not made for 70 cm but with the SWR that low would it hurt the radio in any way?? |
Tech237
Moderator
Username: Tech237
Post Number: 1920
Registered: 4-2004
| | Posted on Monday, May 21, 2018 - 10:15 am: |
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OK. How much of the "low" SWR on 70 is truly a low SWR and how much of it is due to cable loss and the cable being the right length to give an apparent low SWR? What I am saying is, that depending on several factors your SWR may not be what you think it is. Where did you measure the SWR? Right at the antenna or in the shack?
Now, in the past, I have used 2M whips on 70cm -1/4 on 2M is close enough to 3/4 on 70cm BUT they were simple plain whips. On a yagi, other factors come into play, such as element spacing. If you try it, do not expect to get good gain or directional pattern as the spacing will be way off for 70cm.
Personally, I'd connect a multiple of an electrical half-wave of coax (at 70cm) to the beam and retest the SWR before trying it. As an aside, this length of coax will reflect the actually impedance at one end of it to the other.
Tech237
N7AUS
God made me an athiest, who are you to question his wisdom?
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Charliebrown
Advanced Member
Username: Charliebrown
Post Number: 623
Registered: 10-2002
| | Posted on Monday, May 21, 2018 - 8:48 pm: |
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I measured at the radio. The cable is about 36 foot long and the beam is a factory MFJ 2 meter with a built in adjustable matcher . The MFJ beam was adjusted from the factory for two meter but is mostly 1 SWR or less on 70 cm |
Charliebrown
Advanced Member
Username: Charliebrown
Post Number: 624
Registered: 10-2002
| | Posted on Monday, May 21, 2018 - 8:48 pm: |
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I measured at the radio. The cable is about 36 foot long and the beam is a factory MFJ 2 meter with a built in adjustable matcher . |
Tech237
Moderator
Username: Tech237
Post Number: 1921
Registered: 4-2004
| | Posted on Tuesday, May 22, 2018 - 8:56 am: |
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That is almost 21 electrical half-wave lengths (20.81) so that makes the displayed SWR suspect for accuracy in my mind. An electrical half-wave of most 50 ohm coax would be 13.7 inches for 70cm, and for accuracy you really should be using an ODD multiple of that.
Tech237
N7AUS
God made me an athiest, who are you to question his wisdom?
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Charliebrown
Advanced Member
Username: Charliebrown
Post Number: 625
Registered: 10-2002
| | Posted on Tuesday, May 22, 2018 - 3:24 pm: |
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Thank you again for the quick reply TECH 237. So my last question is if I use this as is would it harm the radio or should I just order another beam and buy a dual bander??? ps I don't have the cable right now to change it to odd multiple's so would it work as is for now |
Tech833
Moderator
Username: Tech833
Post Number: 2408
Registered: 8-2002
| | Posted on Saturday, June 09, 2018 - 3:16 pm: |
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A 2m beam might have low SWR at UHF, but it will not radiate well at all. Better off with a coat hanger.
Your radio 'Mythbuster' since 1998
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Charliebrown
Advanced Member
Username: Charliebrown
Post Number: 629
Registered: 10-2002
| | Posted on Monday, June 11, 2018 - 8:13 am: |
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Thank you 833. I have been wondering how you been doing.Hope you and the family are doing well. I know it has been a long time since you had considered writing a book with all the answer's and knowledge you have to help us with but I still wish you would. I for one would buy your book should you decide ever to do this. HAVE A BLESSED DAY MY FRIEND AND THANK YOU FOR YOUR HELP. |
