venerdì 9 maggio 2014

70 MHz mobile antenna, fast and cheap!

In last summer, I make some local QSO on 4 meters band with some hams using my Clansman PRC-351, and therefore I need a mobile antenna. The 4 meters band in Italy is used by very few users (almost only Fire Dept., I believe) and so the antennas for this band are very expensive, more than 60-70 euros for a mobile quarter wave!

Don't worry! I'm going to explain how to modify a 2 meter antenna for 4 meters operations.

You need a 5/8 lambda for 2 meters band. Almost any mobile 2 meters 5/8 antenna is built as shown in the following example:

Choose an antenna built in this way (to be sure, verify that hot wire and the braid of the coax are not in short-circuit).

This antenna is more than 1 meter long whip and a base coil with few turns. The 1/4 lambda in 70 MHz is 1.02 meters, so THIS ANTENNA MAY BE MODIFIED FOR 70 MHz. In order to do this, cut the whip until SWR dips at 70 MHz. Is very simple, and works for ANY 5/8 antenna for 2 meters band.

For the mod, I choose a well-built antenna; a Caletti Atlas, built by the Italian farm Caletti. I buy this antenna from a surplus reseller, is new in box and shipped with base, cable and connector for 7.50 Euro, a real bargain for a quality antenna!

This antenna is yet cutted for f >155 MHz and so is not useful for 2 meters, but is long 109cm (whip only), enough for my purpose. The firt thing is to measure the antenna with an analyzer; Nicola IZ8ITJ and his MFJ259B helps me very much for this job. The SWR, measured wuth the antenna mounted on a magnetic base over my car shows interesting results:

Resonance (3/4 lambda) between 159.9-161.5 MHz 
(SWR < 1.1/1)

Resonance: (1/4 lambda) 55.770 MHz 
(SWR 1.0/1)

in the ham bands:
Frequency       SWR
50 MHz               1.9/1
51 MHz               1.8/1
70 MHz               2.0/1
145 MHz             1.9/1

If you not are a fan of SWR 1/1, this antenna may be usable on 6, 4 and 2 meter with no mod!!!!

But my idea is to build a 4 meter whip, and then began to cut the whip measuring SWR with the MFJ259B. I stop at SWR 1.3. The antenna is:

Lenght: 85 cm
(from the end of the spring to the end of the whip)
Resonamnce (1/4 lambda): 70.200 MHz 
SWR 1.3/1 R=43, X=13

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