Topband: Usable size of Flag antenna and FSM

Fri Feb 26 17:30:26 EST 2021

Great information JC! I finally got my Hi-Z 8 up and running again after many problems. It was and still is a great antenna for RX. I am now using the upgraded preamp at the output of the array as designed by K7TJR.

In addition I have one of your Waller Flags on a 40 foot boom at about 95 feet. It was pulled down last weekend so a new TIC Ring Rotor could be installed in place of the old one. Two broken fiberglass loops have been replaced. It goes back up tomorrow, 27 February. I think this one was #2 or #3 as I recall. Your 40 dB pre-amp really works great. It’s amazing what you can hear when polarization is horizontal. It also excels on the Long Path.

73,  

John, W4NU 
K4JAG (1959 to 1998)

Sent from my iPhone

> On Feb 26, 2021, at 4:44 PM, Don Kirk <wd8dsb at gmail.com> wrote:
> 
> Hi JC,
> 
> Thanks for posting all the great info.
> 
> In my article I mentioned I normally use 20 dB of gain with my portable
> flag, but on 80 and 160 I sometimes use more gain.  I just double checked,
> and sure enough I really do need more than 20 dB of gain on 160 meters when
> using my DX-440 receiver on weaker signals  I just did a rough test on my
> DX-440, and it's sensitivity at 1.8 MHz is considerably lower than it is at
> 4 MHz as an example.  I can just start hearing signals at 1.8 MHz that are
> around -120 dBm (at 4 MHz it's more like -137 dBm), and I think this
> explains why more than 20 dB is useful in my application down on 160
> meters.  If we assume a 500 Hz wide filter, and thermal noise temperature
> of 300 deg K, then I believe the thermal noise generates a -146.84 dBm
> signal to the receivers input.  If we then amplify that signal by 20 dB we
> wind up at -126.84 dBm which is still a little below the level I can detect
> with my receiver at 1.8 MHz.  Tom W8JI mentioned a long time ago that more
> than 20 dB should not be normally required with negative gain antennas
> unless the receivers sensitivity is low, and that appears to be the case in
> my situation (he actually said "40 dB gain in front of a receiver is pure
> fantasy, unless the receiver is dead as a door nail".  Therefore having 30
> dB of gain does indeed come in handy in my application whereas it should
> not be necessary if I were using a receiver with better sensitivity (unless
> I'm looking at this all wrong).  I'm glad I have the available extra gain
> that the DX Engineering preamp offers whereas most receivers would not
> benefit from it.
> 
> I hope this makes sense to everyone.
> 
> 73,
> Don (wd8dsb)
> 
>> On Fri, Feb 26, 2021 at 12:58 PM <n4is at comcast.net> wrote:
>> 
>> 
>> 
>> I would like to share some of my experience with small flags'.
>> 
>> 
>> 
>> The directivity is the same for a large chance in frequency but the gain
>> increase with size.
>> 
>> 
>> 
>> As a reference NX4D started with a single flag  14' high 7' wide , using a
>> 20 db gain preamp. Is was good enough to work 150
>> 
>> countries on 160m, for 80m you can reduce the size by 1/2  and expect the
>> same results. Basically it is a flag like k9AY, EWE, pennant and others
>> 
>> loaded loop, one resistor and one transformer.
>> 
>> 
>> 
>> The RDF is a limitation and Doug phased two loops 14x7 spaced 16ft for a
>> 
>> total boom of 30FT. all fiberglass, and an isolated mast from the tower
>> and,
>> 
>> not portable, I called small Waller Flag. The two 14x7 flag was good
>> enough
>> 
>> to work over 200 countries from a 1/5 acre lot in a subdivision with  a
>> lot of
>> 
>> noise form neighbors. Detuning the TX antenna was a must for good
>> 
>> performance, including working JT1CO direct path over the North pole on
>> 
>> 160m.
>> 
>> 
>> 
>> But the gain of the antenna was a limitation on 160m, and he built a
>> Monster
>> 
>> WF to work 311 DXCC on 160.
>> 
>> 
>> 
>> 2 db NF and 20 db gain is all you need for a vertical  flag or dual flag
>> like the WF(Waller Flag, from Doug Waller, NX4D)
>> 
>> 
>> 
>> 33db gain is too much for a flag or WF vertical, it is good for a flag or
>> a
>> 
>> WF horizontal, and at least 75 ft above ground.
>> 
>> 
>> 
>> Here is important to remember if you S meter is moving you have too much
>> 
>> gain, I match my preamp gain for s0 on band noise during the day. There is
>> a
>> 
>> lot of signals bellow S0. Keep the gain at minimum.
>> 
>> 
>> 
>> To reduce common mode noise a twisted pair 100ohms feed like helps  a lot.
>> 
>> 
>> 
>> Measure noise figure at 1.8 MHz is a great task and care, with a good
>> signal
>> 
>> generator and good sound card, you can take one measure in 2 hours of work
>> 
>> for -+ 0.1 db accuracy.
>> 
>> 
>> 
>> Doug me and Dr Dallas did a long experiment to measure that and a small
>> flag
>> 
>> 14'x7', was not thermal noise limited for DX use on low babnds.
>> 
>> 
>> 
>> At a very quiet rural area with -125 dbm noise floor during the day. For
>> 
>> reference , my city lot average -85 dbm during the day and the best I ever
>> 
>> measured was -95db on those winter Sunday Mornings with light rain.
>> 
>> 
>> 
>> The Dallas files is not available, Dr Dallas is SK but I have some paper I
>> 
>> can share here.
>> 
>> 
>> 
>> So a small loop is not thermal noise limited on 160m, and works very well
>> on
>> 
>> 80, 60 40 and 30m. some big contest station using a HWF reported problems
>> of
>> 
>> power line noise on 20m and the HWF saved them transmitting on the yagis
>> and
>> 
>> receiving on the HWF. On 80 and 40m the HWF is comparable only to a 4
>> 
>> elements full size yagi. ( same directivity, but vertical noise canceling)
>> 
>> 
>> 
>> The easy way and accurate is to use  FSM  easy to do in 3 steps , here >>>
>> 
>> 
>> 
>> https://www.owenduffy.net/software/fsm/index.htm
>> 
>> 
>> 
>> 
>> 
>> FSM (for Field Strength Meter) is a software application that extends a
>> 
>> conventional SSB receiver to allow measurement and calculation of field
>> 
>> strength of radio signals or interference. FSM is a software
>> implementation
>> 
>> of a development of the technique described by Ed Hare of the ARRL in
>> 
>> "Manual Testing of Field-Strength Levels Using Conventional Receivers"
>> dated
>> 
>> August 2004 .
>> 
>> 
>> 
>> You need a good sound car and a 50 ohm shielded load, like an  N or BNC
>> 
>> connector  50 ohm termination.
>> 
>> 
>> 
>> 
>> 
>> Regards
>> 
>> JC
>> 
>> N4IS
>> 
>> 
>> 
>> Sent from Mail <https://go.microsoft.com/fwlink/?LinkId=550986>  for
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>> 
>> 
>> 
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