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Moonraker CB antenne 5/8 (or 1/2 ?)

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#1
Hello,

Bought a Moonraker GAP-F 5/8 wave antenna for 27 MHz.

The antenna is attached to a mast ca. 6 m. above ground and there is nothing around it at least 20m. The cable/connectors are ok and about 10m RG58. The radio is a TTI 1100 4W AM / FM.

After assembly to 6.4 - 6.5 m length I got a scary SWR on 3.5 to 4 ... tried to go back and forth with the last bit but nothing helped. After many attempts, I found that a length of 5.4 - 5.5m helps a lot. On the whole CB band I got a SWR max. 1.4.

The only idea I can come up with is that the antenna is a 1/2 wave not 5/8, or ...?

What's wrong here ?

Thanks in advance,
Daniel
 
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plant city florida
#2
from what i can tell from a picture of it,, it is a base loaded antenna,, since it has no ground plane radials i would advise making sure it is grounded good to the mast and have a ground rod attached to the mast,,,i dont know what type of loading was done,, but it being inside the bottom of antenna i would say it is not made for high wattage use,,, i havent seen one of these up close being i am in usa,,,,so dont go by what i am saying since it is just from seeing the picture and reading the specs,,,,,
 
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#3
A 5/8-wave antenna has a matching coil to convert the impedance to your 50Ω coax. If that coil isn't connected properly you will have high SWR and poor performance.

Most likely the ground connection of the impedance matching coil is broken, and you just made your antenna into a bastardized 1/2-wave antenna which will have significantly less gain.

Fix the coil, and restore the antenna to its original length if you can. Hopefully you didn't cut anything important, or you should trash your mistake and start over.
 
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#6
Hello,

Bought a Moonraker GAP-F 5/8 wave antenna for 27 MHz.

The antenna is attached to a mast ca. 6 m. above ground and there is nothing around it at least 20m. The cable/connectors are ok and about 10m RG58. The radio is a TTI 1100 4W AM / FM.

After assembly to 6.4 - 6.5 m length I got a scary SWR on 3.5 to 4 ... tried to go back and forth with the last bit but nothing helped. After many attempts, I found that a length of 5.4 - 5.5m helps a lot. On the whole CB band I got a SWR max. 1.4.

The only idea I can come up with is that the antenna is a 1/2 wave not 5/8, or ...?

What's wrong here ?

Thanks in advance,
Daniel
It would have to be a Gap 'H' to be the 1/2 wave. Is it marked anywhere with those markings? If you are getting a SWR of 1.4 over the whole band, it is tuned properly.
Larry
 

buddrousa

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#8
I owned a Moonraker 4 and replaced it with a Moonraker 6 back in the late 1970's. People today have no idea how great the avanti antenna line was with the AstroPlane and the Astrobeam antennas.
 
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#10
The Moonraker uses a gama match for tuning. There should not be any coils what so ever.
Have you experimented with the gama match any?
The antenna being discussed is the "Moonraker GAP-F 5/8 wave vertical antenna". While you could fashion something similar to a Gamma Match, the usual matching scheme uses a tapped inductor. The antenna in question does not have a Gamma Match.

Daniel,

I recommend that you go back to the full length and try again. But this time try and make it a little longer, not shorter. The antenna is designed for 26 - 30 MHz. It should give you a reasonable SWR at the low frequency (26 MHz), but if it doesn't, making the antenna longer is the direction to go.

A vertical antenna tends to have a low impedance (20-30 Ohms) so don't be surprised if you can't get lower than 1.5:1. You may possibly need a loading coil to get the input impedance up a little. Look up "vertical antenna loading coils" on Google. They are pretty simple to make and the search will give you some good ideas.

Also, as was previously mentioned, make sure you have a good ground. You mentioned a 6M mast but you didn't say what it is made of. Is the mast grounded?

Do you have access to a antenna analyzer? Maybe a friend or local club? An analyzer will give you a better picture of the antenna's impedance.

Martin - K7MEM
 
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#11
It has a coil and it's DC grounded (probably something like in the diagram picture). Everythings looks ok, contacts, soldering.

On the antenna was nothing like markings, just the alu pipes. On the box was written Moonraker 5/8, 6,5m ...

Thanks,
Daniel
 

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#14
A 5/8-wave antenna needs a ground plane to work properly. You should have 3 or 4 horizontal elements acting as an RF ground for the vertical. If the antenna kit doesn't have them, it's using the mast as the RF ground and you aren't going to get as good RF performance. You'll have issues with the mast not being resonant and probably RF leakage down the mast and your coax shielding and into your electrical wiring.
 
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#16
Hello K7MEM. One of your points was the problem. The ground. The antenna is now up and running with 1,6-1,7 on SWR scale on CB band.

What I wonder is why I get diffrent SWR readings when I measure the power. For example:
26.565 SWR 1:1,5 POWER 7W ??
27.405 SWR 1:1,7 POWER 4,1W
27.990 SWR 1:1,6 POWER 3,8W

The radio is a 4W so where comes those 7W ?

Thanks, Daniel.
 
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#17
You're not reading your meter correctly. You need to put the meter in SWR calibration mode, key the mic, and adjust the SWR calibration so that the meter needle deflects full-scale (but not more than that). THen switch to SWR measure mode, key the mic, and measure the SWR. The wattage reading you get at this point is irrelevant because you are measuring SWR, NOT power.

Conversely, when you are measuring power, you are NOT measuring SWR, and your SWR reading is meaningless.
 
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#18
Hello K7MEM. One of your points was the problem. The ground. The antenna is now up and running with 1,6-1,7 on SWR scale on CB band.

What I wonder is why I get diffrent SWR readings when I measure the power. For example:
26.565 SWR 1:1,5 POWER 7W ??
27.405 SWR 1:1,7 POWER 4,1W
27.990 SWR 1:1,6 POWER 3,8W

The radio is a 4W so where comes those 7W ?
I'm glad you got things working satisfactorily. The SWR reading that you show are probably as good as they are going to get. So there is no real need to go further.

As "jonwienke" pointed out, you may just be reading things incorrectly. It's like any other meter. When your reading SWR, follow instructions and only pay attention to the SWR scales. When your reading Power, only pay attention to the Power scales.

But, to answer your question, it is possible to read more power than your radio is capable of generating.

Generally, your transmitter is closely matched to your coax. But there can be a mismatch at the antenna feed point, which is what is causing your SWR. This mismatch reflects some of your transmitter's energy back down the coax. When this reflected energy reaches the transmitter, it again is reflected. So you lose some forward and reflected power to the coax.

But all is not lost because, that reflected power, when it reaches the transmitter, will again be reflected back to the antenna. This reflection continues, losing power with each trip up and down the coax, until the amount of reflected power is negligible.

But a condition can exist where the coax is of a length such that, the reflected energy, that is coming back from the antenna mismatch, is in phase with the transmitter energy, at the transmitter output connector. Since a power meter really can't tell the difference between the two energies, the in-phase powers can add and give the appearance of outputting more power than the transmitter is capable of.

In the end, the only thing you should be worrying about is that, the output of your transmitter is matched sufficiently to your antenna system (coax and antenna) and does not cause the transmitter to start cutting back on output power. In that situation, all of the power that you apply to the coax, minus the losses in the coax (forward and reflected), will make it to the antenna and be radiated.

Martin - K7MEM
 
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#19
Thanks for explanations.

In this configuration I could hear my home base from roughly 26Km.

I think this is "As good as it gets"...

Daniel
 
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#20
A mobile CB antenna is supposed to use the roof of the car like ground plane, isn't it ?

What happend if I mount it on a grounded (earth) pole with the braid of the coax conected to the pole ?

Is this something someone can use ?

Thanks,
Daniel
 
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