Tuesday, March 17, 2009

Making the Connection

In the aftermath of the very fun ARRL VHF-UHF 2009 Sweepstakes, I am in the process of trying to upgrade my VHF-UHF capabilities. I've got a number of antenna projects planned—yagis, halos, all the good stuff—and, of course, putting more antennas in the air means that you've got to run feedline and make a connection to your rig.

In the past, I would have rather blindly figured that, since they put an SO-239 connector on the back of my rig (even for 2 meter and 70 cm connections) it must be just fine to use that for VHF-UHF stuff. Well, I'm finding out that that is an incredibly naive view.

Many of you may already know this, but the PL-259/SO-239 combination does not maintain a constant impedance as the frequency increases. So down in the HF range, you're probably fine. But as the frequency increases into the VHF and certainly in the UHF range, the connector itself can start giving you an impedance mismatch and the mounting signal losses that go with such mismatches. N connectors maintain a constant impedence at much higher frequencies, hence their usefulness in VHF and especially UHF and microwave applications.

Here's what Chris, KF9OP my microwave guru has to say about the matter:

When I think PL259, I think vacuum tubes, the Ed Sullivan show and the 1950's.

Then there's the post-hippie 1970's BNC option for less than 50 watts and less than 250 MHz. However, BNC was only designed for less than 500 MHz, and its main benefit is ease of connection. If this isn't of value, then I would only use it because of cost/availability.

The 1980's man, uses N for more than 50 watts, SMA for less than 50 watts.

The 1990's man doesn't use connectors, since the 1 dB per connector hit is too great. He solders directly to the PCB ;)

The millennial man doesn't use coax at all! He integrates his radio into his antenna and runs his power/data up the tower. ;)
Now Chris is way out of whack with his estimate of 1 dB per connector; a properly installed connector, of the correct type for the frequency used should be more like 0.1 dB. But that last point is something that Chris and I have discussed a lot. Especially as you go up, up, up in frequency it makes less and less sense to spend big $$$ to try and corral that RF as it goes through a feedline. If you put the transmitter at the antenna you save all that expense and path loss. But since that's not always practical, one does sometimes need to run feedline.

So, back to connectors, there was a good article on the installation of N connectors last year in QST magazine, "Those Type N Coax Connectors," in the April 2008 issue. Unfortunately, it's not on-line yet. But there is a diagram on this here. And there's an interesting exchange about the relative merits of the N connectors in ham radio applications over on EHam.net.

73 DE W9HQ

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