Re: ATU and Bandpass Filter

Posted by donovanf on Jun 24, 2017; 1:55am
URL: http://elecraft.85.s1.nabble.com/ATU-and-Bandpass-Filter-tp7632019p7632046.html

Hi Don,


The technique you use with monoband horizontal dipoles and
bandpass filters is exactly what we do at the W3AO Field Day
site, except we use monoband Yagis rather than dipoles on 40,
20, 15 and 10 meters. (we also use dipoles on 40M).


The recent recommendation in the Elecraft Newsletter about using
two perpendicular horizontal dipoles isn't very effective, and the
concept was incompletely presented. In order to achieve excellent
isolation between two perpendicular dipoles, two conditions must
be met:


- the second dipole needs to be almost exactly perpendicular
to the first dipole, even a five degree error significantly reduces
the isolation.


- most importantly, the second dipole must be perpendicular
to the center of the first dipole. An offset of just a few feet
left or right of center significantly reduces the isolation.


The advantage of this technique is that two horizontal dipoles can
be installed in a physically small space with very high isolation,
but the big disadvantages are that the perpendicular dipoles must
be precisely positioned a nd inevitably at least one of the dipoles
is likely to be oriented to an non-optimum azimuth,


We're fortunate at W3AO to have a 1000 x 200 foot open grass
field for our Field Days, The technique we use with excellent
results is to place our antennas for the same band end-to-end with
300 feet of separation between adjacent antennas. While the
isolation is significantly less than two precisely positioned
perpendicular horizontal dipoles, it has the big advantage of having
both antennas oriented to the same azimuth.


With four 20 meter Yagis sited end-to-end with 300 foot separation
between adjacent antennas we routinely operate four transmitters
(CW SSB RTTY and GOTA) on 20 meters with no trace on interference.


73
Frank
W3LPL




----- Original Message -----

From: "Don Wilhelm" <[hidden email]>
To: [hidden email]
Sent: Saturday, June 24, 2017 12:03:29 AM
Subject: Re: [Elecraft] ATU and Bandpass Filter

What Matt said is true. However, if your antennas are close to
resonance, go ahead and use the ATU to "touch up" the tuning.

If you are trying to use a multiband antenna such as a G5RV and such,
you might as well forget the advantage of the bandpass filter.

Our local club discovered that multiband antennas were a major problem
at multi-transmitter Field Day sites. We now use single band dipoles,
and yes we use a bandpass filter for each of the FD bands.

73,
Don W3FPR

On 6/23/2017 4:35 AM, Matt Maguire wrote:
> The problem is that the filter is designed to work with a 50 ohm characteristic impedance (ie. with a 1:1 VSWR). This means you need to put ATU *after* the filter, not before, otherwise the filter will not work properly.
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