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Drill vs. MagMount ??
All things being equal, is there a performance advantage (in 2meter/70cm
bands) permanently mounting an antenna via a hole in the roof vs. using a MagMount?? How about other bands?? Thanks, Everett |
Hello,
On 2m/70cm some antennas work better when you use a mag mount (Sirio HP2070 for example) because they are tuned to be used with a mag mount: added parasitic capacity between bottom of the mount and the car's steel. Expected you do not drive a 'vette or an Audi... For lower frequencies it's better to drill holes or to properly ground the antenna to the car, as most mag mounts are not wider than a sq foot and you would quickly be annoyed by the potential differencies between the car's ground and the antenna ground, resulting in HF re-entry problems as your antenna will be then antenna + coax cable and all the hassles of bad and/or variable SWR. 73 de F8BOE Olivier ...-.- |
Thanks Oliver, very clear.
73 with gratitude Everett "F8BOE" wrote in message ... Hello, On 2m/70cm some antennas work better when you use a mag mount (Sirio HP2070 for example) because they are tuned to be used with a mag mount: added parasitic capacity between bottom of the mount and the car's steel. Expected you do not drive a 'vette or an Audi... For lower frequencies it's better to drill holes or to properly ground the antenna to the car, as most mag mounts are not wider than a sq foot and you would quickly be annoyed by the potential differencies between the car's ground and the antenna ground, resulting in HF re-entry problems as your antenna will be then antenna + coax cable and all the hassles of bad and/or variable SWR. 73 de F8BOE Olivier ...-.- |
On Fri, 04 Mar 2005 23:51:18 -0800, Everett wrote:
All things being equal, is there a performance advantage (in 2meter/70cm bands) permanently mounting an antenna via a hole in the roof vs. using a MagMount?? How about other bands?? Thanks, Everett Yes a properly installed antenna will out perform a MagMount, always has alway will. |
On Sun, 13 Mar 2005 15:15:50 GMT, Korbin Dallas
wrote: On Fri, 04 Mar 2005 23:51:18 -0800, Everett wrote: All things being equal, is there a performance advantage (in 2meter/70cm bands) permanently mounting an antenna via a hole in the roof vs. using a MagMount?? How about other bands?? Thanks, Everett Yes a properly installed antenna will out perform a MagMount, always has alway will. How does the hole mount compare to the lip-mount that is well grounded? I don't really want to drill a hole in my roof, but I want to get off the magnet and a lipmount on my hatch will leave the antenna at roof-top level. -- 73 for now Buck N4PGW |
Korbin Dallas wrote:
Yes a properly installed antenna will out perform a MagMount, always has alway will. By how much? Why? Roy Lewallen, W7EL |
By how much? Why?
I would assume the direct connection with the roof metal. I tried this one time, after trashing a larson magmount with about 84 knocks off the car... When I converted it over to a hole mount, with the usual NMO, there was a good bit of difference. I didn't measure, as the mag mount was toast, but it was quite large actually. Like maybe a change for say S9 on a machine, to maybe 15-20 over S9. Very noticable in my case using the larson 5/8 antenna on top of a car roof. Was like a different antenna.. MK |
That's very interesting. I can't see how the mag mount could change the
pattern shape any, so your experience indicates a severe loss in the coupling capacitor, maybe as high as 90% or more. That's hard to imagine, so I wonder if anyone else has had a similar experience -- or, much better yet, actual measurements. Roy Lewallen, W7EL wrote: By how much? Why? I would assume the direct connection with the roof metal. I tried this one time, after trashing a larson magmount with about 84 knocks off the car... When I converted it over to a hole mount, with the usual NMO, there was a good bit of difference. I didn't measure, as the mag mount was toast, but it was quite large actually. Like maybe a change for say S9 on a machine, to maybe 15-20 over S9. Very noticable in my case using the larson 5/8 antenna on top of a car roof. Was like a different antenna.. MK |
I can't see how the mag mount could change the
pattern shape any, so your experience indicates a severe loss in the coupling capacitor, maybe as high as 90% or more..... A pretty large amount...I guess it is possible maybe my mag mount performance was down a bit due to it being knocked off the car so many times...Not sure why that would effect it much though..Maybe the connections were getting a bit corroded also...Hard to say really... Probably would be a better test to compare against a brand new one...The one I had was the standard rectangular looking larson mag mount. Looks sorta like a small box with magnets underneath..I do have to say though...When I switched to the hole mount, the increase in performance was instantly noticable, as I knew what all the local machines did as far as strength before the switch. BTW, my comparison was mainly just receive on various repeaters...Being things are reciprical, I never really worried about transmit tests...MK |
The mag mount depends on capacitve coupling to ground while the "drill a
whole in the roof" mount is directly connected to the ground plane. Also, when using a hole in the roof antenna there is no problem withcoaxing being crimped in a door or window, etc. I have used the NMO (Motorola style) in the roof mounts for 2 meters and 450 MHz since 1978 with various vehicles. No problems. IMHO, it is the ONLY way to go. John |
All you say is true. I'm just having trouble envisioning what it is
about the mag mount's effective capacitor that could cause it to dissipate 90% of the power being fed to the antenna. That's one outrageous ESR, even more so considering the size of the effective capacitor. I personally find a mag mount to be convenient, since I use my two meter radio only a few times a year at most, and in two different cars. I'm perfectly willing to take a little hit in performance for the convenience, but not to the tune of 10 dB or more like MK reported. And if there really is any significant performance sacrifice at all, it means that there's something I don't understand and would like to. If there's a 10+ dB difference, there's something I *really* don't understand. The fact that I get a nearly 1:1 SWR with the mag mount makes it even more puzzling -- if the capacitor had an ESR of over 300 ohms (which it would have to in order to drop the signal 10 dB), the SWR would be terrible. But "Korbin Dallas" said: "Yes a properly installed antenna will out perform a MagMount, always has alway will." That's a very positive statement, with no room for exceptions. But so far I haven't seen any reason why it should be true except for MK's single experience that doesn't make sense unless there was some other problem, like damaged coax, with his mag mount. (I can see the possibility for poor performance in an HF mag mount -- but like the OP, I'm talking only about VHF/UHF ones.) I've used a VHF mag mount for over 20 years and have had to replace the coax only once. But then again, I use it very infrequently. Roy Lewallen, W7EL SBC News Server wrote: The mag mount depends on capacitve coupling to ground while the "drill a whole in the roof" mount is directly connected to the ground plane. Also, when using a hole in the roof antenna there is no problem withcoaxing being crimped in a door or window, etc. I have used the NMO (Motorola style) in the roof mounts for 2 meters and 450 MHz since 1978 with various vehicles. No problems. IMHO, it is the ONLY way to go. John |
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