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a question about VHF/UHF antennas?
I have a dual band yaesu (144,440mhz) with 50 watts output. I was planning on installing a single antenna just for 440mhz operation (2m is going to relatively unused for my purposes) I plan to put a half wave dipole up at about 30 feet for 440mhz. I am using #16 copper antenna wire fed with 50 ohm coax (probably rg-58).....i want to shield the antenna from the elements by encasing it in 1/2 inch pvc pipe (i will feed at the center through a small hole in the pipe and cap both ends)........what i am wondering is will thus hurt my performance at all?......the radiating element will be completely encased in pvc...... i know some pvc manufacturers add metal filings to the plastic to meet strength requirements but i will be sure to get pure plastic....so hoe much would this design hurt my performance?
It will probably have some effect on the resonant tuning of the dipole, but how much is hard to say. Just cut the elements a little long and then trim them using an SWR meter for lowest SWR at your center frequency.
I have done this years ago.
But, in my case I used copper pipe as the elements inside the PVC pipe. I did this to help broaden the bandwidth of the antenna so that tuning wasn't critical. Thicker antenna elements help broaden the resonant tuning and reduce radiation resistance.
True enough, at VHF and UHF, RG-8 is a better choice for low loss 50 ohm coax.
However, you should note that a dipole, vertical or horizontal, is typically 75 ohms impedance.
http://en.wikipedia.org/wiki/Dipole_antenna
The easy way to achieve 50 ohms impedence from a dipole is to make it an Inverted-V dipole, in which case it will now be ~ 50 ohms again to match RG-8 coax.
http://www.flashwebhost.com/circuit/antennas_for_ham_transmitters.php
While using PVC encasing, I just used 45 degree elbows for each leg at the feed point and it now creates an Inverted-V (90 degree) angle between the elements.
Quansheng TG-UV2 VHF/UHF Dual-Band HT