Topband: DXE Active Receive antenna question

[Roger D Johnson]

I have had the DX Engineering 4 square array for several years and have made a 
series of
measurements using my vector Network Analyzer (VNA).

By "receiver" I assume you mean the preamp box at the base of the whip antenna. The
preamps are designed to be used with a 102" whip antenna. The input is NOT 50 Ohms
and using a standard signal generator will give erronous readings! You can use a 
50 Ohm
generator but you must make up a dummy antenna circuit that simulates the 102" whip.
I used a 50 Ohm resistor to ground and a series capacitor of about 17pf (from 
memory).
There are formulas around for calculating the proper capacitance of whip antennas.

With no jumpers on the input, the response is pretty flat from BC band up to 30 
MHz or so.
With these type of arrays, the level of output decreases with closer spacing of 
the elements
and with lower frequencies. My array seemd OK on 80m but output was very low on 
160m.
If you connect L1MF, you will get about 6-8db peak on 160m. The output on 80m 
will be the
same but response will drop off below 160m. This may be an advantage if you have 
cross
modulation from strong BC stations in your vicinity.

In the end, I made up 4 21ft elements from old Yagi parts to get what I felt was 
sufficient
signal output on 160m.

73, Roger





On 2/18/2012 8:34 PM, D Rodman MD wrote:
> I should be operating the contest, but spent some of the afternoon
> troubleshooting my DXE circular receive array.  One thing led to another and I
> decided to bench test one of the receivers.  We pulled out my service monitor (RF
> signal source) and played with the jumpers on circuit board while listening to
> the receiver and watching the S meter.  To our surprise the compromise position
> for the system (160&  80m) in terms of jumper position does not produce the best
> receive signal one can obtain.  It looks more like a monoband system rather than
> a duoband.  By changing the jumpers we could obtain at least 10dB signal increase
> on each band separately, but also at the expense of an even greater loss of
> signal on the complimentary low band.  I wonder if anyone else has experimented
> with the active receivers in terms of jumper position on the bench or in the
> field and can comment on this finding.  I have been somewhat frustrated by my
> lack of ability to hear topband signals from time to time and having the
> receivers in a different jumper position might be the trick.  Before I go out and
> start taking things apart in the cold, I thought I would ask around first.
> Thanks for the help.  73.,
>
> David J Rodman, MD
>
> Assistant Clinical Professor
> Department of Ophthalmology
> SUNY/Buffalo
>
> Office 716-857-8654
>
>
>
>
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
>

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