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View Full Version : replaceing 5/8 wave whip with 1/4


rover67
08-21-2014, 04:56 PM
Howdy,

I *think* this is OK but figured I'd post and ask.

I had a 5/8 Larsen wave whip on the back left corner of my FJ60's roof (hatch mount)

It broke off at the base of the whip and I mistakenly ordered a 1/4 whip as a replacement. I wanted a 5/8 wave but messed up... anyways.....

Is it OK to simply throw it on after trimming to 19.5" so it's kinda tuned for the middle of the band or am i missing something?

I know there is more to it but I just need to not fry my radio this weekend. I have no SWR meter and no real time to mess with it so it's either this or the old trusty mag mount as a backup.

(Edit: sorry for the horrible spelling in the title.. eek.....)

DaveInDenver
08-21-2014, 08:22 PM
You need to clarify, are you replacing the whole antenna or just the metal whip itself? You could replace the whole thing, the part that screws onto the NMO if that's what you have, and be good enough.

You can't just interchange the whips. The reason is that a 1/4 wavelength antenna is 36 ohms characteristically, which is close enough. But 1/2 and 5/8 are high impedance and have matching networks to make them appear approximately 50 ohms. So putting a 1/4 whip on a base designed for 5/8 would create a mismatch.

rover67
08-21-2014, 11:19 PM
Ahh see I knew I was missing something... Yes I was just interchanging whips. I totally forgot the base had resistance stuff going on in it to make the shorter antennas work. I'm silly. I think what I'll do is steal the whip off my cb antenna since it's longer and cut it to length so it'll be the same as the old 5/8 wave whip

DaveInDenver
08-22-2014, 05:08 AM
The whips are interchangeable so that would be fine if the diameters are close. For example no issue cutting the whip from a Larsen NMO27 to the same length as your old NMO150. The only way they would not be universal is if there are loading coils on them, which is not typical of mono band antennas but pretty much standard with multi band ones.

CardinalFJ60
08-22-2014, 09:47 AM
I have no SWR meter...

I've got the MFJ SWR meter if you need to borrow it.:thumb:

rover67
08-22-2014, 11:35 AM
That's the case Dave, in fact those are the two antennas I have. I think i'm good to go since the whip from the CB antenna is actually pretty close to right...

http://home.kendra.com/kj7yl/ant.htm


48" looks like my trim length.

DaveInDenver
08-22-2014, 11:45 AM
Your PDF link is to your local computer. Just make sure you follow the cut chart for the NMO150 and not one for a 1/4 wavelength. Also, 5/8 wavelength isn't necessarily the actual length because of that matching network in the base changes the whip length slightly. The antenna is electrically 5/8 wavelength, not necessarily physically. It'll be cut to something like 41".

http://www.pulseelectronics.com/download/3150/nmo_vhf_series_cut_chartspdf/pdf

Other ones:
http://www.pulseelectronics.com/products/antennas/larsen_antenna_products/technical_reference/cut_charts

rover67
08-22-2014, 11:48 AM
yikes thanks!

ok, i see.. I was looking at the wrong part of that chart!