{"id":9157,"date":"2016-08-29T18:12:00","date_gmt":"2016-08-29T22:12:00","guid":{"rendered":"http:\/\/www.engineeringradio.us\/blog\/?p=9157"},"modified":"2023-03-19T11:58:18","modified_gmt":"2023-03-19T15:58:18","slug":"the-isocoupler-and-the-sx2-5","status":"publish","type":"post","link":"https:\/\/www.engineeringradio.us\/blog\/2016\/08\/the-isocoupler-and-the-sx2-5\/","title":{"rendered":"The isocoupler and the SX2.5"},"content":{"rendered":"

The second post in the series, “things to do with a truck body tool box.”<\/p>\n

We have this client who, several years ago, moved their translator to their AM tower. All is well for a few months, then the much beloved Harris SX2.5 transmitter begins burping.\u00a0 The SX2.5 transmitter was of an age when, apparently, VSWR fold-back circuits were just a gleam in Hilmer Swanson’s<\/a> eye.\u00a0 The correct description of the sound made over the air during this event would be “motor boating,” because that is what it sounds like.\u00a0 Obviously, very undesirable.<\/p>\n

Thus, the isocoupler was removed from the tower, dried out, waterproofed, and replaced.\u00a0 That lasted about six months.<\/p>\n

Once again, the isocoupler was removed from the tower, a capacitor was remounted, drain holes and a small vent were added to the top of the unit and it was replaced.\u00a0 That lasted about a year.<\/p>\n

I am getting a little tired of this and so is the client.\u00a0 Time to rethink the entire setup.<\/p>\n

We had several leftover parts from various AM decommissioning over the last few years which included these nifty sample loop isolation coils:<\/p>\n

\"AM<\/a>
AM antenna system sample loop isolation coil<\/figcaption><\/figure>\n

Why not repurpose one of these to make an isocoupler for the translator?<\/p>\n

Enter; the truck body toolbox.\u00a0 This one is slightly smaller than the last one, measuring 23.5 x 18 x 16 inches (60 x 45 x 40.5 cm).<\/p>\n

The isolation coil consists of 35 turns of 3\/8 coax on an 11.5 inch diameter form.\u00a0 The coil length is 15 inches.\u00a0 I calculate the length of the coax on the coil to be out to be right around 100 feet using the \u03c0 x D x (turns) formula.\u00a0 I measured the inductance with my analyzer, which came out to 200 \u03bcH.\u00a0 Not to shabby.<\/p>\n

\"Checking<\/a>
Checking the length of cable with TDR<\/figcaption><\/figure>\n

The coax is Cablewave FCC38-50J which has a velocity factor of .81 and the TDR shows it to be 100 feet also.<\/p>\n

\"Coil<\/a>
Simple coil impedance and reactance<\/figcaption><\/figure>\n

At 860 KHz, the isolation coil presents 1,200 impedance.\u00a0 I don’t think that will be good enough for that cranky old SX2.5.\u00a0 I decided to make a parallel LC circuit (AKA a tank circuit) to bring up the impedance.<\/p>\n

Tank circuit formula:<\/p>\n

\"tank_circuit\"<\/a><\/p>\n

Where:<\/p>\n

FR<\/sub><\/em> = Resonance frequency in Hertz
\nL<\/em> = Inductance in Henrys
\nC<\/em> = Capacitance in Farads<\/p>\n

Given that I have two leftover capacitors, one is a .001 \u03bcF and the other is a .0012 \u03bcF, those values determine where the coil needs to be tapped.\u00a0 I also wanted to have a good bit of coil in the circuit on the tower side before the capacitor tap to dampen any lightning strikes on the tower.\u00a0 Thus the inductance needs to be about 28 \u03bcH.<\/p>\n

Using Wheeler’s coil inductance formula:<\/p>\n

L= (d2<\/sup> x n2<\/sup>)\/(18d+40l)<\/p>\n

where:<\/p>\n

L = inductance in micro Henrys
\nd = coil diameter in inches
\nl = is the coil length in inches
\nn = is the number of turns<\/p>\n

I removed a small portion of the outer jacket on the coil at approximately the 28 \u03bcH point (12 turns) then installed a .0012 \u03bcF capacitor.\u00a0 I used a small variable capacitor to tune for resonance on the carrier frequency.\u00a0 With this setup, at 860 KHz, there is >47,500 impedance.\u00a0 That goes down to about 16,000 ohms +\/- 10 KHz.<\/p>\n

That should make things better.<\/p>\n

Then I mounted the coil and capacitor in the truck body toolbox.\u00a0 There is a fair amount of stray capacitance from the box itself, which raised the resonant frequency by 5 KHz.<\/p>\n

\"Device<\/a>
Device Under Test;\u00a0 initial testing of isocoil after fabrication<\/figcaption><\/figure>\n

Resonance is slightly above the carrier frequency with the permanent fixed .0012 \u03bcF capacitor.\u00a0 I think this will change once the unit is connected to the station ground plane.\u00a0 The network analyzer indicated there is too much capacitance in the circuit.\u00a0 Unfortunately, this may be as good as it gets, however, the analyzer shows the impedances are still pretty high:<\/p>\n\n\n\n\n<\/colgroup>\n\n\n\n\n\n\n\n
Frequency (KHz)<\/td>\nImpedance (Ohms)<\/td>\nDeviation from Carrier (KHz)<\/td>\n<\/tr>\n
850<\/td>\n9,950<\/td>\n– 10<\/td>\n<\/tr>\n
855<\/td>\n14,720<\/td>\n– 5<\/td>\n<\/tr>\n
860<\/td>\n28,590<\/td>\n0<\/td>\n<\/tr>\n
865<\/td>\n59,580<\/td>\n+ 5<\/td>\n<\/tr>\n
870<\/td>\n24,780<\/td>\n+ 10<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The base impedance of this tower is 34 ohms on the carrier frequency, so the isocoupler should be invisible to the transmitter across the 20 KHz occupied bandwidth of the station.<\/p>\n

The FCC38-50J cable has a loss of 1.04 dB per 100 feet at 100 MHz, which is the figure I will use to calculate the insertion loss on the FM translator antenna system.<\/p>\n

The old isocoupler is made with RG-214, but likely a somewhat shorter length.\u00a0 RG-214 cable has a loss of 1.9 dB per 100 feet at 100 MHz.<\/p>\n

Installation:<\/p>\n

\"Isocoil<\/a>
Isocoil mounted on back of ATU<\/figcaption><\/figure>\n
\"Isocoil<\/a>
Isocoil mounted on back of ATU<\/figcaption><\/figure>\n

Before and after measurements with the network analyzer show a very slight change in the reactance at the tower base.\u00a0 Nothing major and easy enough to tune out with the series output inductor of the ATU.<\/p>\n

If I were to do this again, I would simply tap the coil at ten turns from the bottom, measure the inductance and install the proper value capacitor.\u00a0 Since this had to be constructed with the parts on hand, less the truck body toolbox, it because a bit cumbersome to get close to the resonant frequency.<\/p>\n

All this got me thinking; there are other possible uses for such a design.\u00a0 Crossing a base insulator with an Ethernet cable always presents some unique problems.\u00a0 I know the WISP forum that I read, they are always talking about how difficult it is to mount an antenna on an AM tower.\u00a0 What if… armored Cat5e or Cat6 cable was used with waterproof RJ-45 jacks?\u00a0 Something like that could carry Ethernet data and DC voltage past the base insulator to a three or four around sectorized access point and an edge switch or router mounted on the tower.<\/p>\n

\"Armoured<\/a>
Armored category cable specifications<\/figcaption><\/figure>\n

just thinking…<\/p>\n

Anyway, it would not be hard to make coils and install capacitors for the right frequency<\/p>\n","protected":false},"excerpt":{"rendered":"

The second post in the series, “things to do with a truck body tool box.” We have this client who, several years ago, moved their translator to their AM tower. All is well for a few months, then the much beloved Harris SX2.5 transmitter begins burping.\u00a0 The SX2.5 transmitter was of an age when, apparently, … Continue reading The isocoupler and the SX2.5<\/span><\/a><\/p>\n","protected":false},"author":4,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4],"tags":[87,284],"class_list":["post-9157","post","type-post","status-publish","format-standard","hentry","category-tech-stuff","tag-am","tag-am-tower"],"_links":{"self":[{"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/posts\/9157","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/users\/4"}],"replies":[{"embeddable":true,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/comments?post=9157"}],"version-history":[{"count":19,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/posts\/9157\/revisions"}],"predecessor-version":[{"id":11115,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/posts\/9157\/revisions\/11115"}],"wp:attachment":[{"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/media?parent=9157"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/categories?post=9157"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/tags?post=9157"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}