Topband: low angle reception using active antennas

[Andy Ikin]

Trevor G0KTN, Wrote on December 05, 2007 11:20 AM

"Haven't seen any replies to this, perhaps there were none. If you received 
anything "off-list" could you post a summary here please."

Trevor, I have been hoping for more than one response to my enquiry before 
posting a summary.

I have received one off line repsonce from someone who had been very 
gracious to model the two senarios for me.

Basicllay if the input of each active antenna was say 100k shunted by 5pF 
then the mono-pole would have 1.6dB over the dipole with an input of 1M 
shunted by 1pF then the diffence is just 0.1dB. Therefore I don't see 
anything to worry about.

The rational behind the question is;

I need to replace an Active Vertical  that uses a 19dB broadband amplifier 
plus a 25:1 input transformer to "match" a 5m 1.5inch diam Alumuim pole and 
a ground rod for a broadband Phased Array ( 500kHz- 2MHz). Whilst this 
creates a good E field vertical for MW use with IOP2 +90dBm and IOP3 +43dBm, 
the phase match is not close enough for use in an 2 el endfire phased array. 
i.e. if one drives the xmfr/amp  via a 62pF cap to simulate the mast 
capactance from 500kHz to 2MHz, the variation in propagation delay is 
approx. 500ns. Hence, with say a 10 percent ( 50ns ) variation between 
xmfr/amps, this totally screws up the phasing scheme, Currently I am using 
large Aperture loops with this phasing scheme as these are phased matched 
within a few ns. However, I would like to try phased verticals as these 
offer a reduction in high angle skywave at the expense of a much wider 
azimuth and increase in local noise!!

The active mono-pole I am using has a Fet source follower  and a 
complementary emitter follower. Running on 12V, this provides unity gain 
into a 50 Ohms load with IOP2 +74dBm and IOP3 +43dBm ( test sigs 0.8 and 
1MHz via 22pF to simuate the Whip Cap. ). The variation in propagation delay 
from 500kHz to 20MHz is approx. 5ns, an excellent figure.  A problem with 
this design is; when using 2 diodes in series  "reverse biased" for static 
protection, this significantly degrades the IOP2/3. Also the IOP2 is too 
dependant on the correct Fet source follower  bias. Replacing the mono-pole 
with an active dipole using 2 Fet as a differential drain source feedback 
amplifier followed by a balance amplifier, should remove the requirement for 
critical Fet bias.

Since submitting the post to the Reflector. I have parstially resovled the 
IOP problem with the static protection input diodes by fitting 470k across 
the diodes to ensure that they are running in reverse bias. However, this 
fixes the IOP2 degregation but there is still some IOP3 degregation.

73

Andy G8LUG