[TowerTalk] Proximity of Antennas and Multi-Radios

[michael morgan]

Thanks Steve this is great information I just skimmed it but will read it
deeper when I get home from work this evening as well as download Jim's
papers he linked to.

Michael

On Fri, Aug 6, 2010 at 3:17 PM, Steve, W3AHL <w3ahl at att.net> wrote:

>  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
>