... and fairly high isolation exists between Rx and tx (typically 24-dB).

This eliminates use of high power TR relays. One needs typically 60-dB isolation (for power up to 1kW) so a fairly low-power relay can be used on the Rx port to add isolation (typically a mw sma relay). e.g. 1000w (+60 dBm) - 24-dB = +36 dBm (4w). The relay must be able to handle 4w (easy) and have more than 36-dB isolation (also easy). The relay needs very low insertion loss (not easy).

Andy, Both are feeds used on dishes: The scalar ring is really a choke ring and used to control RF fields at the mouth of the horn. It can be used on both septum and non-septum feeds. Typically, it is needed for dishes < f/d = 0.4 (f/d= focal distance/diameter and is a measure of how deep a dish is) High f/d ratios require a horn with wider beamwidth (aka less gain). The VE4MA horn is one that commonly used for f/d<0.4 "Scalar" choke rings are used on the cylindrical "VE4MA" dish to cut off sidelobes on the dish pattern by suppressing radiation on the outside of the mouth of the horn. The basic VE4MA horn is simply a section of cylindrical wave guide with one end closed. On 1296 circular polarization is used. For eme the standard is to transmit in RHCP and receive in LHCP. RH=right hand; LH=left hand and refers to which direction the RF wave rotates. The easiest way to visualize it is by thinking of how a nut rotates on a bolt on RH thread vs LF thread. The reason two senses of CP are required is that CP is reversed upon reflection from the surface of the Moon. To generate a circular polarized signal there are two basic methods: 1) using a hybrid to feed two probes inside a horn at right angles to each other with RF phased by 90-degrees, or 2) using a septum feed horn which has a stepped center plate running down the middle of a section of waveguide (either square or cylindrical shape). This plate is called the septum and it separates two probes inside the horn. One is used for Rx and the other for Tx and the nature of the septum is that is produces circular polarization of both senses but opposite at each probe. There is actually a third method which uses tuned stubes to generate CP. It is very effective but difficult to properly tune. The septum's advantage is its simplicity and high efficiency. In the septum the two polarities are produced without need of external hybrids (which add loss) and fairly high isolation exists between Rx and tx (typically 24-dB). This eliminates use of high power TR relays. One needs typically 60-dB isolation (for power up to 1kW) so a fairly low-power relay can be used on the Rx port to add isolation (typically a mw sma relay). e.g. 1000w (+60 dBm) - 24-dB = +36 dBm (4w). The relay must be able to handle 4w (easy) and have more than 36-dB isolation (also easy). The relay needs very low insertion loss (not easy). For dishes like mine with f/d=0.5 higher gain is needed (smaller beamwidth) to properly illuminate the surface of the dish without spilling over the edge. This is done with either flared horns (like I am using) or something called a dual-mode horn. This is just a simplification of the general issue of dish feeds and the best tutorial is given by W1GHZ's Microwave Antenna Book (on line). 73, Ed - KL7UW At 06:15 AM 12/17/2011, Andy Furlong wrote: > Hi Ed...this is Andy WA2FGK... > I am a /newbie/ on EME...ABOVE 432 MHz > I have 500 watts on 1296 out of a DB6NT Amp and a 12 foot dish. > QUESTION...*WHAT IS THE ADVANTAGE of a Septum Feed....vs > whatever the type of feed is called that has the SCALAR ring? > > *NOT a newbie to EME as we are on 144 MHz(WAS #163) > and 222 MHz with 4 x 7W M^2 and have 6 initials off the moon... > and have worked 432 MHz with 4 18 foot Directive System Antennas, > but now serious about 'movin-on-up' to 1296 and 2304 Mhz. > Would appreciate _any_ info on one feed vs the other. > Thanks and 73 > Andy > > > At 03:30 AM 12/17/2011, you wrote: >> I don't recall who expressly asked for my measurements so I will put >> here for all: >> >> Test equipment: >> K3 + 15 dB atten* + DEMI 1296-28HP => Bird43(10K element) => 1w, >> *needed to reduce output to 1w >> HP432A + 20 dB coaxial atten (10w) >> >> I ran approx 2w into the Septum Tx port (NF) to measure isolation at >> the Rx port (sma-F): >> 1) Reference level adjusted so HP432 reading +10.0 dBm >> 2) Measured = -12 dBm at Rx port, indicating isolation of 22-dB >> Adjustment of tuning stubs did not affect the isolation. >> 3) Input SWR ~ 1.5 (previous measurement of SWR with 60w on the dish >> indicated no measurable SWR) so perhaps may be some reflection inside >> the room? >> >> Next I measured insertion loss and isolation of my sma relay used at >> the Rx port. >> 1) Reference with +30 dBm (1w) indicated on the Bird43. HP432A read >> +7.8 dBm (22.2 dB atten: combination of cables+adapters+20 dB atten). >> 2) Output thru the relay plus short .141 jumper = +7.7 dBm indicating >> insertion loss = 0.1 dB** >> 3) Output on unswitched port of the relay with switched port >> terminated with 5w sma load was unmeasurable on the lowest setting of >> HP432 implying better than 37.7 dB isolation. >> >> I could have run the transverter up to 25w (+43 dBm) to increase >> range of the reading of the HP meter but figured relay isolation >> quite adequate for running 300w into the septum. +24.7 dBm -22dB >> -37.7dB = -34.9 dBm into the preamp. >> >> These measurements were taken with septum off the dish inside the >> house. Once I have the new feed mount bracket constructed and septum >> reinstalled these measurements will be repeated to determine effects >> of reflections from the dish. Placing my hand over the mouth of the >> septum showed increase power reflected into the Rx port showing >> lessening of isolation a couple dB as expected. >> >> **Measurement accuracy probably not better than 0.1 dB. >> >> >> >> 73, Ed - KL7UW, WD2XSH/45 >> ====================================== >> BP40IQ 500 KHz - 10-GHz www.kl7uw.com <http://www.kl7uw.com/> >> EME: 50-1.1kw?, 144-1.4kw, 432-QRT, 1296-?, 3400-? >> DUBUS Magazine USA Rep dubususa(a)gmail.com >> ====================================== >> >> _______________________________________________ >> Microwave mailing list >> microwave(a)lists.valinet.com >> http://lists.valinet.com/cgi-bin/mailman/listinfo/microwave > > > 73, Ed - KL7UW, WD2XSH/45 > ====================================== > BP40IQ 500 KHz - 10-GHz www.kl7uw.com > <http://www.kl7uw.com/>EME: 50-1.1kw?, 144-1.4kw, 432-QRT, 1296-?, > 3400-? > DUBUS Magazine USA Rep dubususa(a)gmail.com > ====================================== > > > > _______________________________________________ > Moon-Net posting and subscription instructions are at > http://www.nlsa.com/nets/moon-net-help.html >