Discone vs Diamond X50a for VHF?

[chrisdm123]

I recently set up an SDR (RSPDuo) and am really enjoying it for scanning. I am currently using a Diamond X50a that I have repurposed from an old ham base station install, fed with 50 feet of RG213A to the roof of my 2 story house (this is as high as I can go, but the apex of my roof is about 35 feet so I have some decent height).. I'm only interested in VHF/UHF, and mostly with county fire frequencies in the 155Mhz range... Also some distant amateur repeaters both 2m and 70cm... I would like to improve performance to try and grab some more distant stations in 3 directions (so a directional antenna will not work)... Particularly potential fire/air tanker traffic (155Mhz) on adjacent mountains up to 150 miles away (West and South)...

I hear everyone recommending a discone for SDR's... But in looking at specs of discones like the Diamond D3000, it shows 2Db of gain. My Diamond X50 specs show 4.5Db of gain on 2 meters... So would a discone actually improve my reception of distant stations even though the specs show lower gain?

Thanks in advance,
Chris KX7D/WROJ613

 

[AK9R]

You can find the installation instructions for the X50 here. In those instructions are the SWR plots for this antenna on the 2m and 70cm amateur radio bands. You will note that the SWR plots are not flat. IOW, the antenna is tuned to work in the amateur bands and may not perform well outside the amateur bands.

 

[chrisdm123]

Thank you, but if you'll read my post again my question is not about the installation of an X50, as I already have it installed and have tested the SWR. The question relates to the application of the gain ratings vs a discone antenna.

 

[AK9R]

Diamond often does not state the reference for their gain claims. In reality, it's usually decibels relative to a theoretical isotropic radiator. IOW, that 4.5 dB of gain is 4.5 dBi which is roughly equivalent to 2.4 dBd (referenced to a dipole). Diamond also claims that the gain of their discone is 2 dBi so it's roughly the same as a dipole.

Diamond's quoted gain for the X50 is in the amateur radio bands. The gain outside those bands may be the same or it may not.

If you are looking for a gain antenna in the 155 MHz range, you'll probably have to consider an antenna made for the VHF public safety or business bands.

 

[prcguy]

If you just need reception in the 155MHz range Comet and maybe Diamond make commercial versions of their amateur antennas that will favor 155MHz, otherwise the amateur version will be performing below published specifications. Where an amateur or commercial version of an X50 or similar antenna will work ok across a narrow range of frequencies, a typical scanner Discone like Diamond D130J or D3000 will give roughly 1/4 wave ground plane performance from around 100MHz to at least 500MHz then the radiation pattern starts to go down the toilet going higher in frequency.

If you want to monitor something other than 155MHz like VHF or UHF air, UHF commercial, etc, a Discone will probably outperform an amateur version X50 on VHF/UHF air and other frequencies far away from the amateur bands.

 

[chrisdm123]

Thank you. I just ordered the Comet GP6C, high gain dual band commercial vertical.