| Author |
Message |
    
Glassbysam
| | Posted on Wednesday, April 09, 2003 - 7:14 pm: | 
|
I know I have always heard that this cannot be done, but I am tempted to try it...4 element vertical beam and using an I-MAX 2000 on the same mast extending above the beam boom about a foot....
has anyone tried this , or has every body just heard that it cannot be done???
will the fiberglass coated antenna make a difference??
I know that I can turn the 4 element horizonal and it will work ....
waiting on comments
thanks
sam |
Stepchild
| | Posted on Wednesday, April 09, 2003 - 8:42 pm: |
|
Glassbysam,I with you.I've wanted to try that with my MR4 but,everbody has told me that it won't work because antenna's need to be at least a wave length apart(36ft).If you try it let us know the results either way.Stepchild |
DeadlyEyes
| | Posted on Wednesday, April 09, 2003 - 9:31 pm: |
|
Easy answer....
If the antennas are not at least 1/2 wave length appart there will be linkage of some kind most definately.
A spacing of one full wavelength or as Step stated at the very least 36 feet is required to prevent one antenna from screwing up the radiation pattern of the the other antenna. |
Sixkiller505
| | Posted on Wednesday, April 09, 2003 - 10:24 pm: |
|
hey guys!! you are late!! on the east coast where i live we have been doing his for years.( i started radio in 1970 and i saw it here then)it seems to work fine for the people who use it,nobody has complained about swrs being high,and they sound the same etc,etc< 505 love country |
CM 3885
| | Posted on Wednesday, April 09, 2003 - 10:40 pm: |
|
Ive seen guys do that but ive always went by the 36' rule.. Any antenna within 36' will affect the beams performance.... |
de
| | Posted on Thursday, April 10, 2003 - 8:29 am: |
|
Re Sixie....
Linkage does not only effect SWR. Linkage can also screw up the radiation pattern and the radiation angle sending a lobe of signal here it should not be. This linkage is the principle behind the way a beam works. |
Simon
| | Posted on Thursday, April 10, 2003 - 12:49 pm: |
|
been doing it for years (well I was when in Australia) just put AT LEAST 1/4 wave between the two anntenna (minimizes coupling from one to the other). |
Czar
| | Posted on Thursday, April 10, 2003 - 1:39 pm: |
|
We have also done this for years with no problems whatsoever.I run an IMAX on the top of a 6 element beam{flatside position}and it gets out great,also the beam under the IMAX provides a good ground plane for it.You may have to set swr a little differant than if there was no IMAX there but thats about it,no other problems should be found doing it this way.Also hams have done it this way for years too,the put 2 meter,6 meter,etc. verticles above their beams with no problems. |
Tech833
| | Posted on Saturday, April 12, 2003 - 1:15 am: |
|
If the yagi (beam) is horizontal, this will work fantastic. Spacing is not an issue at all.
However, if the yagi is vertically polarized, then the Imax and the beam will both be degraded.
The reason is complicated, but imagine your Imax alone mounted in between two large metal objects. |
CM 3885
| | Posted on Saturday, April 12, 2003 - 9:52 am: |
|
My uncle ran a moonraker 4 with an GP antenna mounted just above it and he had problems with it acting like another driver element and it made the beam ineffective in rejection. He took it down and put it on his chimney.. |
Simon
| | Posted on Saturday, April 12, 2003 - 10:06 am: |
|
Tech833
Actually spacing is important - not so much as to prevent pattern or SWR distortion but to reduce the amount of RF from one antenna being coupled back into the second. The second antenna can 9under the wrong circumstances) re-radiate this energy in such as way that it causes RFI. One of the more common RFI problems in a ham or other multi antenna, multi radio shack is rf energy being re-radiated by either unterminated antennas or antennas connected to radios that are not turned on. In the latter case it is the protection diode across the antenna connector that is the main culpret. |
Tech833
| | Posted on Saturday, April 12, 2003 - 11:46 am: |
|
3885- The Moonraker is not a true quad, and a vertical mast or antenna through the elements will degrade the performance greatly.
Simon- If the beam is horizontal, then spacing is not important. In a gamma fed yagi, the entire antenna is at ground potential, and will look like a ground plane to the vertical antenna. Any RF coupled into the yagi from the vertical will be very, very minimal. If any RF is induced into the yagi, it will mostly be re-radiated as if the yagi was being fed with a very small amount of power. This will not cause TVI.
What 'protection diode'? I am not aware of any such component in any of my receivers. The amount of RF coming down the cable of the yagi induced by the vertical would be in the order of 20 or 30 dB/m according to my calculations. This is so minimal that it would not harm even the least expensive of receiver designs.
Of course, adding spacing by multiple wavelengths would reduce the induced current in the yagi below the vertical, but that spacing would have to be great to reduce the RF by even a small degree.
Most antenna switches provide enough isolation that any RF coming down the cable from the yagi would itself be undetectable when measuring it while in the field produced by the vertical. |
Bigbob
| | Posted on Saturday, April 12, 2003 - 2:58 pm: |
|
TRY that with a quad,he he. |
CM 3885
| | Posted on Saturday, April 12, 2003 - 3:54 pm: |
|
I know its not a true quad but ive been using mine for 5 years and i like it besides no one can tell the diff between that and a true quad on CB anyways..... |
Tech833
| | Posted on Saturday, April 12, 2003 - 9:53 pm: |
|
3885, I was not at all suggesting that the Moonraker was somehow a poor antenna, only suggesting that running a mast through it will degrade the performance while doing the same with a true quad will not.
There is no argument that the Moonraker is a good antenna. I prefer them to the V quads. If I was to mount a vertical omni antenna over a dual polarity beam, I would choose a quad to make it possible.
Bigbob, try what? |
Simon
| | Posted on Sunday, April 13, 2003 - 10:33 am: |
|
Tech833
try measuring it and I'd be surprised if its not a lot greater than that. Take a look at your radios and unless they are extremely old they are liable to have a diode across the antenna connector or more likely on the receive side of the rx/tx switching circuit to shortout high RF voltages. Every commerial radio I have worked on in the last 25- 28 years has had such a diode (I mean Icom, Yaesu, Philips, Motorola, Pearce simpson, Uniden etc).
As for the small amount of power being re-radiated from the yagi not causing RFI I suggest you read back issues of 73 AR, QST and CQ as many articles have been printed over the years that indicate that such a thing does happen and my own experiences (several at ham shacks (not mine), two at repeater sites (commercial ones) and at least one at an Emergency Services radio base) all have shown that this is possible and does in fact happen. Maybe you have just been lucky not to have come across it - so far. |
Galileo
| | Posted on Sunday, April 13, 2003 - 3:59 pm: |
|
There will be no prob as long as the beam is in the horizonal position, as a matter of fact you will probably get a groundplane effect for the Imax with this setup... |
Glassbysam
| | Posted on Sunday, April 13, 2003 - 4:43 pm: |
|
ok
it is up ...maco 4 element vertical 45' to the boom...6' above the boom of the beam I mounted the IMAX 2000, so the IMAX is 74' above the ground..... SWR on beam is 1.4 and SWR on IMAX is 1.2...I can hear locally as well on the IMAX as I can on the beam...but the IMAX is 20' higher than the beam ....the beam hears just the same [meter reading ] as the groundplane but does cut some noise from the back.....
I have turned the mast while transmitting on the groundplane and have been told that the signal fluctuates as the mast is turned......receiving signals do NOT fluctuate as I turn the mast when listening on the groundplane.....
so the jury is still out and I am continuing to assess the setup...I can actually hear local traffic [40+ miles away ] better than I have ever before....
I may turn the beam horizontal in the next week , just to see how things react...
later
sam |
CM 3885
| | Posted on Sunday, April 13, 2003 - 7:57 pm: |
|
Beams are sweet right?:-) I like to use my 'rakers to talk local long distance to guys 60 to 100 miles away and mobiles 75 miles away.. |
Tech833
| | Posted on Monday, April 14, 2003 - 11:00 am: |
|
Simon,
Hams are not the ultimate authority on antennas.
If you assembled your beam with less than perfect electrical connections, then there is a possibility of creating TVI with an excited near-field vertical. However, this would happen with ANY metal in the field, not just another antenna. If the beam and vertical are solid (like they burnished the connections and used Penetrox) then no TVI or intermod will be generated. That is the truth.
As a 20 year professional engineer, I bump antlers with hams all the time about their poor practices and what passes for an 'expert'. That is why I spend my online time here on the CB forums rather than on ham forums at other websites. Most of the published articles I have written have been for professional broadcasting and two-way professional magazines and books. I steer clear of the hams, they frustrate me. |
Simon
| | Posted on Tuesday, April 15, 2003 - 9:40 am: |
|
Tech833
Re-read my messages as I asaid I have over 25 years in the trade PROFESSIONALLY not just as a ham including designing two-way radio systems for Emergency Services. Trust me, my antenna systems are properly built and maintained. By the way as for distance not being important did you consider that as most verticals work better when mounted around 1/4 wave above ground (halfwave is even better in most cases) that the vertical should be mounted that far above the beam as the beam is now the ground for the vertical?? |
Tech833
| | Posted on Tuesday, April 15, 2003 - 10:20 am: |
|
Simon,
I have glanced at some of the scematics for two way radio I have and do not see a 'protection diode' anywhere near the antenna jack. It seems to me that even doing this with a zener would be O.K. for receive but as soon as you transmit, would clamp and make your tx load a short.
Is this something that occurs after the tx/rx switching circuitry by chance? In one Motorola HT schematic, I see something that may work as a 'protection diode' as you suggest. However, it's actual funtion appears to be a DC return for tx/rx switch current.
Can you post a diagram of a schematic with this component in circuit? |
Czar
| | Posted on Tuesday, April 15, 2003 - 5:50 pm: |
|
verticles work best when ground mounted and having a ground radial system in place.If you have a beam in the HORIZONTAL position, you cam mount the IMAX abot 1 foot above it without any radiation pattern distortion and or swr problem.Verticle mounted 1/4 wave or 1/2 wave above ground may work well for local ,BUT do not work very well for talking dx.
|
Bigbob
| | Posted on Tuesday, April 15, 2003 - 8:30 pm: |
|
Tech833 just what you said,you can do it with a true quad but not a vertical yagi. |
Crafter
| | Posted on Wednesday, April 16, 2003 - 2:25 pm: |
|
Tech 833 I think what simon is talking about is like the bulb on the input side of the yeasu 101. As for rf coming in there, you havnt lived next to me. I used to light up my neigbors radio and move his wattmeter.
Now the beam thing I used a antron in my moonrakers for years never had a problem but it does effect it recieve and radiation pattern but not enough to effect my signal locally I always ran my amp anyway. I used my antron for local and moonraker for DX. |
|
