I'm not sure how useful a lower gain antenna would be in the case being discussed. A lower gain antenna could actually increase the amount of signal received from other antennas above or below at the same site, because the antenna's pattern null wouldn't be as deep, while at the same time producing less gain at the horizon, where the desired signal is presumeably intended to be sent / received.
In cases where the intermod is a product of two signals well off the center frequency of the antenna connected to the "accidental mixer," the pattern of the antenna might look very different at those other frequencies from what it would be at the antenna's design frequency. I've modeled colinear gain verticals in EZNEC, and if you take them more than a few per cent off the design frequency, the elevation patterns get pretty goofy. 73, Paul, AE4KR ----- Original Message ----- From: Mark Harrison To: [email protected] Sent: Wednesday, February 13, 2008 4:15 PM Subject: RE: [Repeater-Builder] Re: Interference on a 6 meter repeater Hi Skipp, I was only suggesting a lower gain antenna as an alternative to using a power attenuator. It will work if all the sources for the IM mix come from external sources, and won't be nearly as useful if the mix frequencies include your repeater TX and you are using a duplex antenna system. I've certainly experienced IM problems when a low gain antenna has been upgraded to a high gain antenna. It's been due the signal strength of the two adjacent off-channel signals increasing. As you say, filtering doesn't help there because they are adjacent channels, but some attenuation is enough to reduce the signals below the overload point of the receiver. And yes, an attenuator in front of the receiver in a duplex repeater helps more than a reduction in antenna gain because you are reducing the local TX signal too. No doubt there are situations where, say, a 6dB attenuator and a 6dB higher gain antenna would kill off IM with no loss in repeater sensitivity. Cheers, Mark VK3BYY > -----Original Message----- > From: [email protected] > [mailto:[EMAIL PROTECTED] On Behalf Of skipp025 > Sent: Thursday, 14 February 2008 4:23 AM > To: [email protected] > Subject: [Repeater-Builder] Re: Interference on a 6 meter repeater > > > Mark Harrison <[EMAIL PROTECTED]> wrote: > > Hi Skipp and Joe, .... > > BTW, You could also use a lower gain antenna instead of a power > > attenuator. High gain antennas are not always a good idea :-) > > 73, > > Mark vk3byy > > In 97.5% plus examples an antenna replacement wouldn't solve > the problem. You could try antennas and cavity filter setups > until the cows come home and this radio would still be > problematic. > > cheers, > s. >
