Necessity For 1/4 Wavelength Length(s) ?

[BOBRR]

Hello,

Will be purchasing a GRE 500, which apparently has a range of about 30 MHz to 1.2 GHz.

If I obtain a telescopic antenna for the unit, and wish to stick with just this antenna realizing it will always be sort of a compromise, the following question arises:

If I leave the antenna extended to its longest length, will this "automatically" provide me the best possible broadband coverage on "all" the frequencies (particularly at the low end), or is this long length negated by the fact that the antenna wouldn't be tuned to exactly 1/4 wavelength most of the time ?

I guess the question would relate mainly to what happens on the really low 30 - 50 MHz frequencies.

Must the antenna in that range "really" be tuned to exactly a 1/4 wavelenth to be effective, or you gain so much by having a quite long length, that exact tuning
isn't all that necessary ?

Thanks,
Bob

 

[Astro25]

It will be somewhat improved if it's at or near the 1/4 length for the desired frequency usage.

IE - It's gonna play better on 460.000MHz if it's at ~ 6 inches, rather than at full length (whatever that may be, unless it becomes UHF 1/2 wave at that length....)

 

[SkipSanders]

There are more than the 1/4 wave resonances available, longer than 1/4 wave may be resonant at 3/4 wave, 1 1/4 wave, 1 3/4 wave, and 2 1/4 wave, also (theoretical 50 ohm resonance)

I have a Diamond SRH789 telescoping antenna (sma connector) for my BC396D, and here's the resonance chart I figured for it (by measuring it carefully, and using an antenna analyzer program to simulate it). I figured in terms of 1/4 segment lengths to make it handy to use without a tape measure 'in the field' as long as I have the chart handy.

Telescoping Antenna Resonances and sizes

Segment 1: .19685m, 6mm
Segment 2: .12859m, 5mm
Segment 3: .12462m, 4mm
Segment 4: .12065m, 3mm
Segment 5: .117478m, 2.5mm
Segment 6: .11281m, 1.5mm

Total Length all open: .801 m (31.535 in.)

1.00 Segment: 356 MHz
1.25 Segment: 308, 942 MHz
1.50 Segment: 274, 828 MHz
1.75 Segment: 244, 740 MHz
2.00 Segment: 222, 668 MHz
2.25 Segment: 202, 610 MHz
2.50 Segment: 188, 564, 946 MHz
2.75 Segment: 174, 522, 876 MHz
3.00 Segment: 164, 486, 814 MHz
3.25 Segment: 154, 456, 764 MHz
3.50 Segment: 146, 432, 722 MHz
3.75 Segment: 138, 408, 632, 956 MHz
4.00 Segment: 130, 386, 644, 906 MHz
4.25 Segment: 124, 366, 614, 862 MHz
4.50 Segment: 118, 350, 586, 822 MHz
4.75 Segment: 114, 334, 560, 786 MHz
5.00 Segment: 110, 320, 536, 754, 970 MHz
5.25 Segment: 106, 308, 516, 724, 934 MHz
5.50 Segment: 102, 298, 498, 698, 898 MHz
5.75 Segment: 098, 288, 480, 672, 864 MHz
6.00 Segment: 096, 278, 464, 658, 834 MHz

Resonances (by calculation) show at ¼, ¾, 1 ¼, 1 ¾, and 2 ¼ wavelengths, for 50 ohm.
As if Center Fed .5, 1.5, 2.5, 3.5, or 4.5 wave antennas, against ground reflection.