[TowerTalk] Proximity of Antennas and Multi-Radios

[Steve, W3AHL]

I just did an analysis of this for our club's Field Day site for next year, since we had problems running 10A with only 5 watts this year.  Someone forgot the bandpass filters (BPF) used in the past, but the concern was which radios created more of a problem with out-of-band interference and how much of a BPF was needed.  In our case a BPF with 60 dB of attenuation is adequate.  The I.C.E. BPF filters used at some stations previously are only 30 dB typically, which doesn't cut it for our use. 

The main problem is broadband noise, which is reported in the ARRL equipment reviews as composite noise.  They only report the level out to 1 MHz from the carrier (22 KHz in reviews before about 2006), but the noise covers 3-30 MHz fairly evenly, depending on the radio.   This noise is a combination of phase noise from the DDS, PLL or VFO and amplitude noise from the driver and PA, primarily.  There may be peaks slightly higher than the average composite noise level, but they are narrow and can be worked around.

I measured the composite noise of six radios used by the club, primarily for the case of transmitting at 14.1 MHz and measuring the out-of-band composite noise at 80, 40, 15 and 10M and at 1 MHz from the carrier to compare with the ARRL lab report.  The best performer was the Elecraft K3 at -160 dBc/Hz, an IC-7000 was -120 dBc/Hz and an IC-718 was the worst at -112 dBc/Hz.  Your radios appear to be in the -130 dBc/Hz range.

To translate that into something more meaningful:
  1.. The noise power received is a function of filter width. For a 500 Hz filter -130 dBc/Hz = -103 dBc.  For a 2500 Hz filter, -96 dBc. 
  2.. dBc is dB relative to the carrier power, which for 100 watts is 50 dBm.  So -96 dBc = -46 dBm.  This is the power being transmitted to the antenna.   
  3.. The power received by the other antenna is a function of both antennas' gain, pattern and isolation due to distance.   In your case we can't assume isolation due to pattern and the freespace loss is probably less than 25 dB at a distance of less than 100'
  4.. So your RX antenna will see about (-46 -25) = -71 dBm.  An S9 is typically -73 dBm.  So you have a problem.
  5.. Ideally you would like to reduce the out-of-band noise to less than -120 dBm.  That would require a BPF with (-120 - -71) = 49 dB attenuation in the bands of interest.   A 60 dB filter would be ideal.
  6.. Such a filter needs to be able to withstand the TX power of the radio it is attached to and have reasonable insertion loss at the center frequency.  And of course you need one for each band you planned to operate on simultaneously, connected to the correct radio.   QST has had several articles on building such filters, but it helps to have access to an analyzer to tune them.  They can probably be purchased.  Since our club is just running 5 watts, building the filters isn't a big deal.
That was probably more detail than you wanted to know!

Steve, W3AHL

Date: Fri, 6 Aug 2010 13:19:14 -0500
From: michael morgan <cmorgan13 at gmail.com>
Subject: [TowerTalk] Proximity of Antennas and Multi-Radios

I have been blessed in the last couple of years and have been able to get
two nice HF rigs (Yaesu FT-2000 and Kenwood TS-2000).  I am making some
changes to my antenna installation and have a question about necessary
isolation if needed.  I am a armchair contestor, I enjoy getting on the air
for contests but typically just for a few hours and in that environment I
would only use one rig.  But what I would like to do is be able to have my
main rig say on 20 RTTY and try to put the Kenwood on 30 or 17.  I have no
intentions of operating on the same band.

For antennas I have a 35' Rohn 25 tower bracketed to back of my house.  I am
going to replace my Cushcraft A3S with a Explorer 14 with the 40M kit at the
top and possibly put a Cushcraft A3WS above it (any advice on a suitable
mast and separation).  Then I have a Gap Challenger Vertical about 40' from
the base of tower.  Also will replace my G5RV with a Dipole for 80/60.

I do have a Heathkit HL-2200 Amplifier but operate 99% of the time
barefoot.  Just turn it on sometimes during a Phone contest or to get in on
a DX station.

Any guidance or advice would be much appreciated.

73's and God Bless

Michael, AA5SH